Is it relevant that your friend is kind & supportive to you at this time - you mentioned ishoos with an other friend (OF) Is this a mutual friend?
I wonder if the OH is put out by yr friend’s loyalty to you, maybe he feels neglected or ignored, maybe his OH is beyond his locus of control?
Is theOF known in any way to this couple?
I have a sense that the OH is looking to make it hard for you to turn to your friend/ has some tie-in with the OF/is a creep who relies on your vulnerability to harass or wind you up/knows you won’t dob him in to your friend as she means much to you (delete as required).
Yr friend says he is manipulative, so you know he has form & is adept at this. He sounds a nasty piece of work & this may not be the only skirmish you have with him.
I suggest you keep you powder dry at this stage, not mention it to your friend right now & prepare to get tough with him if he comes near you.
Screen shot the thread, email images to an account & leave the msg unopened - audit trail. Get that vile thread off your phone, as while it is there, he is living in your head rent free - but make sure you back it up first for audit trail.
Get a voice recorder app on your phone & have it running anytime he is near you. He is most likely to make fly-by comments than have a clear cut conversation with you.
If you need to speak on the subject again to him, tell him you’ll take any inappropriate threads, not to his wife, but to the police as he is harassing you.
It is NOT you, it is him.