I've read that HPDs are easily influenced by others. I think the final blow to my relationship with my HPD was his spending time with a certain person who (I think) is morally bankrupt.
The person is married but having an affair. He also is one of the top bosses in my HPD's company. This person (I'll call him 'slimy') goes on trips with his girlfriend and my HPD and a couple other guys who are subordinates in his company. On top of it all, his girlfriend is not content to just be there with someone else's husband; she has to climb all over all the guys. I think 'slimy' is using my HPD and the other guys as an alibi. If his wife asks about the trip, he'll just talk about going with the guys. (I've also been invited, but learned the hard way not to go on these trips.)
My HPD is suckered by this guy. The guy buys them all drinks and strokes their egos. My HPD thinks this guy really likes him and goes out of his way to do favors for him. I think the guy just uses him.
Anyway, after my HPD has spent time with this guy, it's like he's a different person for a while. The "sleaze" seems to wear off on him. That had actually happened when we had the huge fight that was probably the end of our relationship.
Of course, my HPD is responsible for his behavior, not anyone else. Most people would see "slimy" for what he is, but my HPD is too flattered and too naive to see it.
Frustrating, but as usual, nothing to be done about it.