Can a u.s soldier join the British army?

Of coarse he would and because his American he'd have to get a green card there which he can obtain be marrying you are go threw the long process of becoming a citizen there if he was part of a commonwealth country like Canada he could just move there and join up and get a free citizen ship but his American and there is now way in hell he can transfer to there armed forces NO WAY IN HELL it's immposibke