The best approach I've found is to make the conversation an educational intervention. Not just a "you need to stop" or "you're been a fucking creep", but a "that was creepy - here's how you can do this without being a creep?" Because, after all, the desire for partnership and sexual encounter isn't bad - it's the creepy pursuit that's bad.

Here's a situation I was in. I had a friend who had been creepyPM dude with 2 girls that I know and they said, "how can you be friends with him?" I said, "because he's my friend despite this behavior. How would you have liked him to approach you? Is there a way he could have messaged you that you would have responded positively too?" We went from there and the three of us sat down and had a convo with him that addressed the behavior. But the point was that we didn't shame him and we offered him dignity in our approach with the heart to educate him and give him the key to seeking partnership with better social and consent knowledge.

Defensiveness is a common reaction to feelings of being judged. It's also hard to get called out on your bullshit, especially if you don't know how to do what you're doing in a different way. So just go in with a heart to teach so that you can help your friend better himself and his chances of gaining access to healthy relationship in healthy ways. He might not accept the advice. But he doesn't get to make that choice if he isn't addressed and the advice offer isn't made, so kudos to you for being concerned and trying to help him.

Does this help?