We don't know if the mentioned person's mother tongue is English. They may well not know that MILF is mother I like to f*ck. I'm from a non English speaking country and I can say that I haven't met anybody who'd know that. I myself didn't know that until I read a Wikipedia article. The OP should first try to see if this is the case, otherwise fire them. Some are bad at flirting, and sometimes you say silly things. We shouldn't be this generous at calling everything sexual harassment.