Can a student apply for a new PhD (in the US or UK) while he is currently enrolled into a PhD (in the US)? A physics student is currently enrolled in a PhD program in the US, 1.5 years into yet. After few months he will have received his MSc degree and will start working on his thesis. While his physics department is relatively strong, he is unhappy (perhaps even disappointed) because his very sub-field of interest is particularly weak and researchers in this sub-field very rarely publish decent papers. 

He's seriously considering starting again from scratch and apply for a new place where people are stronger at his field. Can he apply while still being enrolled in his PhD?

How much negativity does the fact that he plans to drop the current PhD add to his application?

Is it better to apply to UK universities, since in US he will be required to repeat the coursework?
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.