Let's review. What you are trying to express is:
John lost his friend. John was crying for his friend.
Of course, such repetition is undesirable, so English has a tool you can use, called a relative-pronoun. Relative pronouns include who, whom, that, and which. Of those choices, who and whom can be used to refer to people. Who is used for subjects; whom is used for objects.
You probably know all that already, but just need advice in applying the rules.
The pronoun you want is the one in the objective case (whom), because John lost his friend. (Using the subjective case pronoun who would mean that the friend lost John, which is not what you want.)
Remember, also, that relative pronouns help you refer to a noun that would otherwise be repeated. Relative pronouns don't stand alone. The correct sentence should be:
John was crying for the friend whom he lost.
That said, even native English speakers often fail to distinguish between subjective and objective cases correctly. Therefore, you have to be careful about parroting patterns that you see and hear, as you may be copying incorrect examples. On the other hand, you are also unlikely to be stigmatized for choosing the wrong case in everyday speech.
Short answer: The word "in" is idiomatic with corner, so I see nothing wrong with the author's wording.
Longer answer:
I was taught that you should say at the top-right corner of something
I don't know where you learned that, but always be skeptical of a simple rule that always tells you which preposition is right – particularly if you think any other preposition would be wrong. These little words are more flexible than a gymnast!
Even without the word corner, I think I prefer in over at or to, although none of these seem "incorrect":
The logo is a tiny picture that sits in the top left.
The logo is a tiny picture that sits at the top left.
The logo is a tiny picture that sits to the top left.
The only 100% accurate, hard-and-fast rule is that there is no 100% accurate, hard-and-fast rule. I can use several prepositions to accurately describe the locations in this picture:
- The logo goes at the top left
- The site name goes at the top beneath the logo
- The site branding goes in the center at the top
- The site utilities go in the top-right corner
- The navigation bar goes below the header and above the body
- The section navigation goes in the body to the left
However, these are okay, too:
- The logo goes on top and to the left
- The site name goes in the header below the logo
- The site branding goes in the middle above the navigation bar
- The site utilities go at the top and to the right
- The navigation bar goes between the header and the body
- The section navigation goes at the left of the body
Best Answer
Technically, in questions we can use either who or whom when it is interpreted as the object of a verb or preposition. However, it is quite unusual to use whom as an interrogative pronoun in modern English except in very formal writing. (It is is much more common to see whom as a pronoun in relative clauses)
The following questions are quite unusual:
We are much more likely, even in formal situations, to say:
However, there is one situation where it is actually ungrammatical to use interrogative who and where we must use interrogative whom. This is when the word whom is the Complement of a preposition at the beginning of the clause:
The first example above is ungrammatical because the word who is the Complement of the preposition by. Because of this, we need to use the second example above, which uses whom.
However, we only have to use whom if the preposition is at the beginning of the question. If we don't put the preposition directly before the wh- word at the beginning of the clause but leave it at the end, then we do not have to use whom. We can use who instead:
In the examples above, the preposition by occurs at the end of the question. We interpret the word who/m as the Complement of this preposition, but the preposition is not at the beginning of the clause. For this reason both of the examples above are perfectly grammatical. It is more natural in spoken English to put the preposition at the end of the question rather than at the beginning.
In modern English, the example with who is far more natural than the example with whom.