I'd urge him to consider carefully not only because dropping out of university might not be wise, but because he'll probably end up in the US on an H-1B visa.
An H-1B visa isn't a big deal, but it prevents you from working for a few weeks on contract to try things out, and it makes it quite a bit harder to change companies or quit.
Valley companies tend to overwork and under-compensate, and an H-1B makes it -that much- harder to get out of a terrible situation if you find yourself in one. An American designer in the Valley can say "no thanks" and change careers or move elsewhere with a minimum of effort compared to the issues associated with leaving with an H1B.
EDIT: It turns out I don't know much about visas and it's hard to get an H1B without a degree (thanks HN - learning a lot!). See replies for a wide variety of speculation as to which visa type would work in this situation, and feel free to sub in your choice of visa where I initially said H1B, because I think the danger of a poor working situation is either similar or even more pronounced with another type of visa.