Maybe. I think it's best to give it a chance at least.
I agree. As soon as a theist can demonstrate some evidence for his deity's existence... well, I may not convert on the spot, given the plethora of simpler explanations (human hoaxers, super-powered alien teenagers, stuff like that), but at least I'd take his religion much more seriously. This is why I mentioned the prayer studies in my original comment.
Unfortunately, so far, no one managed to provide this level of evidence. For example, a Mormon friend of mine claimed that their Prophet can see the future. I told him that if the Prophet could predict the next 1000 rolls of a fair six-sided die, he could launch a hitherto unprecedented wave of atheist conversions to Mormonism. I know that I personally would probably hop on board (once alien teenagers and whatnot were taken out of the equation somehow). That's all it would take -- roll a die 1000 times, save a million souls in one fell swoop.
I'm still waiting for the Prophet to get back to me...
This one is a classic Sunday School answer. The God I was raised with doesn't do that sort of thing very often because it defeats the purpose of faith, and knowledge of God is not the one simple requirement for many versions of heaven. It is necessary, they say, to learn to believe on your own. Those who are convinced by a manifestation alone will not remain faithful very long. There's always another explanation. So yes, you're right, God (assuming Mormonism is true for a moment, as your friend does) could do that, but it wouldn't do the world much good in the end.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.