My boyfriend participated at Henley, won London head last year, and placed 2nd twice in a row at Nationals in a Quad.
Is it better for him to become a successful rower in America or England?
UK has good facilities, but getting the right support can be quite difficult and quite expensive, and if heard of olympic hopefuls not being able to get good sponsorship or good access to good rowing centres, training programs and staff, who then either have to pay for it themselves or if the cant afford it have to brush such an opportunity aside.
I cannot comment on the US standard of training; they seem to be very good as there are a good many US teams that compete at Henley every year. What I can say, is the access to support is greater as relatively speaking, "crew" in the US is a bigger sport than "rowing" in the UK, especially at uni level and beyond, despite rowing often being perhaps considered as "quintessentially English". There is also greater access to funding and sponsorship for sport all round in the US, due to greater private investment than in the UK.
Generally, a handful of friends and friends of friends athletes who have made the transition to the US have done quite well, but I appreciate this isn't always the case. I'm assuming that your bf competes at a very high level, so he probably wouldn't have too much trouble over there, but that's a decision you need to make. Good luck.