Reintroducing after a fight

So for context about a year ago my bfs ex-friends brought their cat over to his apartment for a “playdate” (without mentioning it beforehand) and he let them in which lead to our cats having a fight and he gave the girl cat to me to keep for a while, well now we’re moving in together and shes in the bathroom while we try to reintroduce but i would really like some advice. The girl cat seems scared of the boy and shes hissing and swatting at him under the door, and he retaliates and hisses back, will the fear go down overtime of being in the same vicinity? They loved each other before this mess and theyre 2 and 3 years old if that helps, thank you in advance!