For newbies, weed can make people react differently to it and to a small degree it can lower inhibitions and even make someone a little more horny, but it's very unlikely it's to a level where bad decisions like this are made. The more common reactions, besides the munchies, are negative things like paranoia and nausea/dizziness from taking too much. Either one of those and going to random peoples houses and sex is completely off the table.

She probably knew this person beforehand, had a level of attraction, and was having fun which led her to a place where she cheated physically. Weed is just an excuse for her to not have to account for her behavior.