I've been Christian for over a year, and although a short time, I am strong in this faith. He fell away and now wants to convert. He comes to me for support and I don't know what to say. I don't agree with it. What would you do?Is he asking for your opinion? If so then you should probably give it to him. But it won't help your friend to say you think Mormonism is bogus because your current faith tells you so. Of course your church thinks his church is wrong.
No, the only way I can see for your friendship to continue is to either not talk about it at all, or agree to investigate the truth claims of BOTH your church and his together. And keep in mind the right answer could end up being "none of the above." If you're not ready to face that possibility, I'd go with not talking about it at all.