people buy and sell accounts still all the time - if your partner's account had a nice/desirable username, was built up with trophies/stats in any way, had nice pets or any reason really. if it was easy for someone to gain access to it could have been sold or traded around until someone liked it enough to make it their own! sorry that happened to their account, in a way I feel like something like that is worse than it just being gutted and left alone since now it probably wouldn't even feel like theirs anymore if they got it back.