Learn English – Metro Station or Subway Station in US

The correct term is Afghan although Afghani is often used as well.

The wiktionary entry¹ for Afghani reads:

Afghani

1. A citizen or native of Afghanistan. From an Afghan point of view this name is wrongly being used for Afghans. After the Soviet invasion of Afghanistan millions of Afghans took refuge in neighboring Pakistan. The Pakistanis and the international aid agencies coined this word to speak of Pakistanis versus the Afghans².
2. (usually lowercase) A monetary currency used in Afghanistan, divided into 100 pul.³

Purists will advocate the use of the former (Afghan) while well meaning ethnically minded people will often prefer the latter (Afghani) with no offensive connotation at all. Here is a randomly picked example.

I can see at least two reasons why Afghani could be perceived as offensive.

• Its resemblance with Paki, a deliberately derogatory term for Pakistani.

• The expression "Afghani Arab".  Afghanis are these Muslim fighters (mujahideen) from Arab countries who joined the various resistance movements in the last two decades of Afghan wars. In this sense, the final 'i' is the Arabic suffix marking an origin. The Arabic word for Afghan is also pronounced afġān (افغان). This is probably the meaning that your friend had in mind.

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Note 1:
Wiktionary is actually missing a third meaning: Afghani being also a lesser used synonym for the Pashtu language.
Note 2:
To be understood as "The Pakistanis and the international aid agencies coined this word to speak of Pakistanis [Afghans] versus the Afghans [who stayed in Afghanistan]".
Note 3:
Because of the succession of governments in Afghanistan, Afghanis banknotes are often redesigned and are both affordable and appreciated by banknotes collectors. My preferred one.

The main difference is the use of objects. 'Told' can only be used as a transitive verb. 'Said' can be used either way, but is most commonly used in the intransitive form. Personally I've only seen the transitive form used with a direct quotation, as illustrated below.

I told him the moon was made of cheese.

I said the moon was made of cheese.

"The moon is made of cheese," he said to Mary.

I would never say, 'I said to him the moon was made of cheese.' It may be grammatically correct, but it's awkward.

The only other difference is that 'told' has an idiom:

I told you so!

You would never say: "I said so to you!" You might in some cases say: "I said so!" but it doesn't carry the same tone of rebuke.