It's a good thing you talked your friend down from creating something. The world has too many creators as it is. Don't they know that everything good has been created already? If it hasn't been done then it isn't possible!
The particular app in question has been tried a million times and countless hours have been invested into it, perhaps learning from that body of work would be helpful? But hey, maybe that guy is The One who will come out of nowhere and solve the problem in a completely new manner (half-serious - it's technically possible, but I don't think it's useful to put our eggs in that basket).
His friend would have made a lot of personal progress in their understanding and abilities.
General rule of thumb, if your friend wants to achieve something, help them do it. Don't sit there telling them why they're going to fail. It almost doesn't matter what it is. They want to start a business, be a brain surgeon, jump the grand canyon on a motorcycle, whatever, believe in them and help them succeed. If you don't believe in them, what kind of friend are you really?
I do believe that there is an amazing HTML/CSS design tool waiting to be created. I don't know what it is, and I don't know if that guy would have been the one to create it, but I love to see people try. He probably would have learned a lot even if he failed.
That negative attitude just really gets my goat in real life. There's one guy at work on whom I can count to shit all over any idea I bring him. There's another guy who always gets excited about trying new things. Guess which one I would happily give a recommendation, and which one I wouldn't care if he fell out a third story window?