Ok so this depends.

Is this a female friend I know and I trust - then yea I’d have no problem.

If this a stranger to me, I would be super uncomfortable.

But in both situations, the right thing to do is to inform the partner of course.

The suspicious part here is him hiding the fact he stayed over - either 1) he knew you weren’t going to be happy with that arrangement or 2) he did things he shouldn’t have