Would the person who dumps the dumper also be called a dumper, or rather a dumpster?

The one who is doing the dumping is the ex girlfriend dumper.

The one being dumped is the dumpee.

As my Old Pappy always said, neither a dumper nor a dumpee be. “Dump” is a rude term anyway, I always preferred the phrase “Break it off”. Had to do that a few times back in the day, but only because the woman was moving too fast. Nothing personal, I just always seemed to hook up with females who wanted a ring on the finger pronto, and I just I wasn’t prepared for that.

Dumpee.

Dumping is equal-opportunity with regard to gender.

As @Nomore_lockout ‘s Old Pappy always said, it’s a rude way of thinking and talking.

Dumpster is cute but a mixed metaphor (unless you could get some people to understand that you mean it like “trickster”, but I don’t think you’ll have much luck with that unless you explain it to everyone first), since the common meaning of actual dumpsters are receptacles of garbage, which are passive and get dumped out by dump trucks. So perhaps those who dump are like dump trucks.

It’s not a great term in the first place, even without Old Pappy’s advice. But someone who dumps is a dumper, as @elbanditoroso already answered. And no, that doesn’t mean “because they can’t live up to the expectations of the other”.

Calling the dumpee a “dumper” is something some people say (meaning someone who ought to be dumped) but that’s confusing.

Perhaps the whole term should be dumped. ;-)

It was all a bit dumb, maybe…