What should I say my friend, when he can't decide, what religion he should believe?

Honestly if I was in your shoes the first thing I'd tell said friend is: I can't help you choose a religion. It is up to you to develop an independent mind and figure out what do you truly believe in. And you shouldn't allow anyone including me tell what to believe or not believe. if you choose to be an atheist or something out of the choices you've listed already. It would be still your choice first and foremost.

And that would be the conselhos I'd give them. I believe strongly in the right to think independently. Things like religion isn't something you decide based on advice. It's something you decide based on your own opinion and what you truly believe in. if she can't think I'll suggest her to read books based on both religions constantly to get a better idea about them.

I would say, "I'm sorry. I can't help you decide what religion to follow. That's something you decide on your own. Besides, you shouldn't rush. You don't have to choose a religion at eighteen. You don't even have to choose one at all. Not everyone follows a religion."

Why does she even have to choose she can believe in both if she wants to.

I'm not sure about Buddhism, but I know that with the different sects of Christianity, she has many people she could talk to, Bibles to read, and information based on the Bible. I know in this area (and many places) the Jehovah Witnesses would be happy to do a Bible study with her - in her own início or a place that she would like to study.

After she has enough information on the religions - she could make a decision for herself. I guess the best conselhos I could give is to look into the two religions she has to choose from, and then make her own decision.