This is JMO, but I wouldn't allow it at all. I'd also be exercising the dickens out of your young female without the older male involved, since he'll want to keep up and overdo it, as you've seen. If she's fine when you're around, then just don't leave them alone together. I'm sure they'll work it out, and I'm sure there are others that will advise you to let them work it out. But I wouldn't.