Here are the sentences with the correct pronoun forms:
- Our friend is very happy.
- In spring her friends... (This sentence seems incomplete. Without more context, I can't complete it.)
- Whose cat is it? - It's mine.
- Their address is on... (This sentence also seems incomplete.)
- (my/mine), but the red bag is hers.
- We live in the city. It... (This sentence also seems incomplete.)