OTOH, maybe he will find out he really loves cycling and found the encouragement in this AIDS ride (which I'm assuming is a supported ride) to do something he never thought he could do.

You've told him what you thought, and he didn't listen. Is he your friend?

Sometimes we just have to sit by and let our friends do what they want and offer support if they fail. Not 'I told you so's. They are adults after all.

Yes, 575 miles in 7 days is a lot, especially for someone who just bought a bike and has never cycled before. It sounds near impossible to me, and I have a feeling he'll realize this on about Day 1 of the ride, but he'll be realizing this FOR HIMSELF and not with someone telling him what he can't do (everytime someone tells me what I can't do, I know that I am that much more determined to do it). But also keep in mind that there are people out there that just decide to do something, and surprisingly enough, do it.

Why don't you use your experience to try and help your friend prepare for this, go on some training rides with him, instead of simply naysaying him.