they would have dated sometime in the past year or two and been borderline serious (not quite living together level but heavy feelings?) and broken up for any number of reasons, xander is the type to go all-in with relationships so if this was overwhelming or they weren't on the same page that could have certainly been an issue. it would be amicable though, he's not a petty person and doesn't like to let go of people he cares about so he'd want to stay friends