hey there,
In “my own experience”, a man will tell you what you want to hear in order to avoid conflict or dealing with an issue. This would be a big issue, sooo, yes he could be lying or not.
My advice? Decide now to forgive him if you intend on staying with him. You may never know for sure, so you need to forgive. AND ..ask him to get tested for sexually transmitted diseases. Just for your own peace of mind and protection.
Have you asked this girl any details about the alleged “sleep-over”? She may be able to provide details that back up her claim or if she is pressed for more info and can’t provide it, she may come out as a liar. I would ask though.
Good luck,
Melissa