I flagged the answer left in this case, but the answer has now been deleted and the user left behind. And more generally is there a better way of doing this?
User in question is this one.
If they have a post you can flag, flag it.
If they have a deleted post and you have more than 10k rep, flag that deleted post. If they have a post which isn't deleted, you can flag that - even if it's going to be deleted. Custom mod flags will remain until handled by a mod; they won't be automatically cleared by post deletion like NAA/VLQ flags will.
Otherwise, flag any post to explain the situation.
That could be the question where an answer from the rudely-named user was posted, as @TheLethalCarrot suggested. Or it could be literally any post you want. All moderator flags go into the same queue, so it doesn't matter where you flag as long as you make clear what the issue is, e.g. by linking to the rudely-named user's profile. You could flag one of your own posts, one of a moderator's posts, a recently active post on the home page, post number 123456, ... whatever, it shouldn't matter.
The best way is to flag the answer and link to the user explaining that you believe they should be nuked or renamed. If the answer is deleted before you get a chance to flag it you can then flag the question the answer was on and link to the user again. If the initial post was a question you can just flag any other post you want really, it shouldn't matter too much. I tend to pick one of my own posts but I don't think it matters much.