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.