I’m a mod on a different forum, and our software will flag new members who share an IP address or email with another account. We’ve only had a few instances where someone has tried to create an alternate username, and we slapped their hand and told them if they did it again they would be banned. They were known pot stirrers who were likely trying to create conflict. Regardless, it straightened them up.
I think a message from a mod would be in order to see if they can get the other side of the story. If they don’t respond or refuse to make good on their arrangement I would be getting rid of them. However, this isn’t my forum, and I think the mods here do a fine job of managing the group.