Oops - check the spelling:
My friend works in/for an insurance company.
a) What kind of company does your friend work for?
b) Which company does your friend work for?
(By the way, I would use the preposition 'for' in these cases).
Question (a) would definitely be the correct one here, because you are quite clearly asking what sort of company this person's friend is employed with.
If you wanted to apply question (b) to an appropriate statement, then you would write something like My friend works for Mutual of Omaha, because here you are interested in finding out the specific name of the company.
I hope this helps.
Have a good weekend!