Want to become Muslim?

There is this European guy in my class, I've known him for about 2.5 years and he's mostly a classmate as I hardly hang out with him. He's been going out with a Muslim girl in my class also for the past 2.5 years. About 2 months ago he told me he wants to convert to Islam and wonders if I could help him. He said he's been thinking of it over the past year and thinks its the right thing to do. I'll skip the details about the relationship as I don't think its relevant. Thing is its very likely he's doing this just for the girl. When he first told me I was like, OK how about you think more about it before you convert properly, ya know since there is no coming back. I took him to the mosque with another friend and we taught him to pray. I tried giving him some more information from Islamic sites and asked him to read up before he does it.

Anyway just last week he tells me he wants to convert properly, saying shahadah and all. I suggested its better to do it in the mosque, he showed me a piece of paper with the shahadah written on it and says he knows what to do. So plan is to take him tomorrow (Saturday) to mosque so he can do it all properly.

I am not sure how to feel about that. I am not sure if I am the right person to help out with this stuff as I don't find myself mentally nor spiritually in a position to be a positive influence. Plus it worries me that I can be responsible, what if in a few years time he's not with the girl and he's like "why am I muslim again?" agh, and the whole shebang about apostates.
Just today we're playing soccer and my Iranian friend was talking to him and when he found out he's becoming Muslim he went nuts "are you serious? do you know what you're doing? Aliii this guy wants to become Muslim," etc... He said it as if the guy had lost his mind. I joked about how if we lost one (Iranian friend) we gained another one (European friend).

Another thing is how hmm this sorta happened to me before. "C" asked me to help her becoming Muslim as well. I had the same approach saying she should really think it through and should take her time thinking it over. She became Muslim with the help of some female Muslims and I wasn't really much involved. But she always said I was a large influence. Just under a year later I saw her again and she was saying how she doesn't feel it anymore and wasn't sure about her decision, I was like "a...a....." aggghhh... Well it's been 3.5 years and from what I can tell she's still Muslim...

Bottom line, why do people approach me for such matters? Agh there is another person as well but his story is different. I think people trust me too easily, just 2 weeks ago this girl I know told me I can stay with her parents when she found out I'm moving to Wellington, when I called her nuts and how she hardly knows me she's like "I don't know, you sound very nice and truthful".... riiiight you don't know me matey, don't let my charm fool ya Muhahhahaha