He wants to sleep around? Fine, as long as he's using protection, there's not much to worry about.
However, sooner or later, things will catch up with him, they always do.
Example: I used to live in a group home. A friend of mine who lived there as well, exuded confidence and charm when it came to picking up women. As a result, he was almost never without a date.
One day, while he was out, a woman he had been with a few times came by, was told he wasn't in, but decided to wait for him.
Some time later, a woman he had been with a few times came by to, was told he wasn't in, but decided to wait for him, not more than 3 feet from the first woman!
Neither one spoke to the other, and after a while, they both left, but at different times, and only after leaving a message for my friend at the office.
A short while later, my friend appeared, was told of the circumstance that occurred while he was out, with yet another woman, and actually breathed a sigh of relief.
He confided in me that what had happened while he was out was something he tended to fear once in a while. I told him that I envisioned his "end" when two women are waiting for him in the lobby, one is upstairs at his room, another in the dining room, two more coming down the street from opposite directions, and while he's walking in with another woman!
So first, make sure your friend is protecting himself and his partner, whoever she is at the moment, and second, sometimes it's best if he finds out for himself as to whether or not "karma" will catch up with him.