Is it "ok" to say Allahu Akbar to a muslim friend?

It is ok !

Depends on your friend dynamic. If you have to ask if its okay, then it probably isn't going to be okay.

Do you really believe the god of Islam is good?

No, it's rude. Don't say something unless you truly mean it. That phrase implies you are a Muslim. If you are not a Muslim, you ought not to say it. I would think your friend would be amused at your lack of understanding, and yes, probably offended as well.

You should get creative in how you talk with your friend. There are a lot of other thoughtful things you can say, rather than falling back on an old formula you don't really understand, nor have a right to say in the first place.

yes.
just make sure he is wearing suicide belt with bombs.

So you don't know what it means but you want to say it?

Why?

It is between you and your friend.I I won't recommend to say it loud in public