Can a student apply for a new PhD (in the US or UK) while he is currently enrolled into a PhD (in the US)?
It is possible (provided you meet the requirements of the second institution), but the more important question is whether it is advisable. In the general case, switching to a different university halfway through your PhD is something that you should contemplate only if staying at your current university will significantly adversely affect your career prospects or your personal life. It looks like this is the case for your student, as he wants to study an area of physics under-represented in his current university. 

The issue of whether he should apply to UK universities to avoid having to go through the basic coursework again is irrelevant, I think. If he feels so strongly about switching locations exclusively on the strength of programs, then he should also be ready to graduate a couple years later. Although some students think doctoral programs have an excessive course load, going through the advanced basics of the field is unbelievably useful.