Was the vet or one of his techs male? Is she like this with other males? Assumpta really dislikes women, and I've never been sure if it was a preexisting condition or the fact that my neighbor's wife found her and gave her blood tests and her first shots at home (she's a vet tech)...but Assumpta does NOT like women (I am apparently an exception to the rule, because she always runs to me when she's upset or sick). Bad luck her vet is also a woman.
To this day, whenever my neighbor's wife walks into the house, Assumpta turns into Boongo, the Avenging Death-Kitty of Doom. I feel bad, because Assumpta is the only living creature in the universe who doesn't like this woman, and I think her feelings are a little hurt, because the cat adores her husband...
(She also loathes my mother-in-law, and will just sit and hiss at her from the top of the stairs...I do not discourage this behavior, needless to say
Try having your boyfriend avoid direct eye contact, and try to keep his body turned side to the cat. He can also try blinking slowly and yawning to show he's no threat...usually, the more you try to make friends with a hostile cat, the more threatening you appear, and the more hostile the cat becomes. It's going to have to be her idea to investigate him when she's darn well good and ready, so all he can do is to be nonthreatening and try to "ignore" her as much as possible.