They are both grammatically correct.
With the same consideration in both cases is an adverbial prepositional phrase modifying the verb treat.
That I would a friend is an adjectival clause modifying consideration. The verb in the clause is implied, and in full form would be that I would treat a friend. While the preposition could be added along with the verb, it is not essential. If it were added, the possibilities are
Am I treating this stranger with the same consideration that I would treat a friend with?
Am I treating this stranger with the same consideration with which I would treat a friend?
While both are grammatically correct (assuming you have surrendered on the avoidance of terminal prepositions), they sound clumsy.
The originals sound fine.
There are four factors which decide whether a relative pronoun (or the word that) can be omitted or not:
- Is it a defining relative clause?
- Does the main verb in the relative clause have a separate Subject?
- Is the relative pronoun the first word in the relative phrase? (or is it preceded by another word, for example a preposition)
- Is the word who, which or that?
If the answer to the questions above is yes then the relative pronoun can be omitted. There are lots of duff websites around which might tell you that you can only omit these words if they represent the Object of the relative clause. This is hogwash. You can omit the pronoun as long as it is not the Subject of the matrix verb in the relative clause. The pronoun can, for example, be omitted if it is the Object of a preposition or Subject of another clause embedded within the relative clause.
Here are some examples to illustrate each point. An asterisk, *, denotes an ungrammatical example.
We cannot usually drop a pronoun from a non-defining relative clause:
- The agent I met up with wants you to phone him. (defining)
- *Your father, I met up with yesterday, wants you to call him. (non-defining)
We cannot drop the relative pronoun if the main verb in the relative clause does not have a separate Subject:
- I don't like the elephant you bit. (matrix verb in r-clause has a Subject, you)
- I don't like the elephant you said bit you. (matrix verb in r-clause has a Subject, you)
- *I don't like the elephant bit you. (verb in r-clause has no Subject)
Notice that in the second example the missing pronoun represents the Subject of the verb bit.
If the pronoun is embedded within another phrase, for example a preposition phrase, then it cannot be omitted:
- That's the circus I work in.
- That's the circus in which I work
- *That's the circus in I work.
We can drop the pronouns who, which and the word that, but we cannot drop the pronoun whose:
- That's the table I bought.
- That's the girl I like.
- That's the girl whose table I like.
- *That's the girl table I like.
The Original Poster's example
"For someone used to the tiny creatures we get in England it was something of a shock."
The word someone here has been post-modified by an adjective phrase. Some people argue that this is the result of removing who is from a relative clause. If you have a defining relative clause which uses the verb BE, you can often drop the relative pronoun and the verb BE. Whether it is now a kind of relative clause, or just an adjective phrase, or participle phrase modifying the noun is up for debate. Here are some more examples:
- The man [who was] going into the chip shop was an undercover agent.
- The elephant [who was] interested in the buns was rather plump.
Here's an example where you can't:
- The people who were blond preferred detective fiction.
- *The people blond preferred detective fiction.
We often cannot do this if what's left of the clause is only one adjective.
The sentence has a real relative clause modifying the phrase tiny creatures:
- ... creatures [which] we get in England
This is a defining relative clause, and the verb get has its own Subject, the word we. There are no other words preceding which. We can therefore happily drop the relative pronoun.
We could rewrite the sentence like this to show where potential words have been omitted:
For someone who was used to the tiny creatures that we get in England it was something of a shock.
Note:
Many grammars use the terms restrictive/non-restrictive or integrated/supplementary to describe what I've called defining and non-defining relative clauses.
Best Answer
Shoe has already answered. Here is some additional information that may or may not clarify things further.
The first important thing to understand is that the relative pronoun is part of the relative clause, not of the main clause. Recently, even some native English speakers seem to be getting confused about this, as some weird new practices around who/whom show.
Let's look at some examples around the main clause "The man is here." I will put the relative clauses in bold face.
We can see that in all examples the relative clause immediately follows the part of speech (here the noun man) which it defines or explains; the remainder of the main clause follows after the end of the relative clause. This is not absolutely necessary, but it's the normal order. (One could also say "The man is here who wrote the message", but that's unusual.)
Next we see that in all examples the relative pronoun (in this case who[m]) begins the relative clause, possibly together with a preposition. This can be explained as one of the few remnants that English has from the original proto-Germanic word order: Word order was generally very free, and the sentence started with the semantically most important part of speech. In a relative clause that's the relative pronoun -- where applicable with its preposition. (It's a general phenomenon in Germanic languages that the word order in subordinate clauses is more conservative than that in declarative main clauses.) As a special feature of English, the preposition can optionally be separated and 'stranded' at the end of the sentence (or here: of the relative clause) as in the last example.
If we replace who/whom by he/him and write all the relative clauses as if they were main clauses, we get declarative main clauses with an unusual (almost obsolete) word order that puts enormous stress on [to] him. The only exception is the first example, where the word order happens to be the normal one because it's the subject that needs stressing.
Preposition stranding is a normal but optional and somewhat controversial feature of English. (Controversial among native speakers, that is. Non-native speakers get drills in preposition stranding so they can apply it properly.) I can see two problems with it. 1. It defers the information about the precise role of the relative pronoun in the relative clause to the very end of the relative clause. 2. There are transitions between prepositions and case markings. In fact, English to and of, just like French de and à, have developed to a stage where they can be considered as case markers. (Most case markers in European languages were once postpositions and therefore turned into endings. But prefixes marking case aren't unheard-of, either.) Seen this way, preposition stranding (in case of to or of) separates a relative pronoun from its case marker. That's like separating the -m from whom and 'stranding' it at the end of the sentence.