Namechange as the background of this would be recognisable to anyone in RL.
ExP and I have stayed very, very good friends since splitting almost 4 years ago.
last month he recieved extremely bad news about his fathers health, he asked if i would go (along with our son) to his fathers home country. Did so that day and he very sadly died within days.
I found xp's behaviour very difficult to understand and it made an awful situation so much worse, he had been seeing someone for about 5 weeks and spent the run uo to his fathers death and the week or so after glued to his phone. It felt like we were in the way and I was struggling to understand why he wanted us there at all.
Tried to grin and bear it but came to a head after he let slip he had asked her to come out too, when she couldnt he was going to return to uk 5 days after his father died so he could keep a pre-arranged 'date' with her. I lost it with him at this point as i had been asking if we (me and ds) could return as I was worried about school and feeling very uncomfortable and drained byeverything there. He said no, or rather made me feel terrible for wanting to leave, so we stayed.
He agreed it would be completely wrong for him to leave, his mum needed him and we were there supposedly to support him.
It also became very clear that the nature of their conversations and constant messaging was not about support or any deep feelings they had for each other (which I could understand) but it was pretty much about sex 
grubby pictures, her relaying stories about threesomes, nights outs etc.
Huge fight ensued, i felt i didnt have a clue who this man was. he then seemed to really get how it looked, how their behaviour was making a mockery of the awful situation we were all trying to deal with. He was full of remorse and anger at himself and at her. Said he was using it as an escape and could see now that she absolutely expoited his weak position because she craves attention constantly. He told her to back off (they had been messaging constantly every day, talking and video calling every chance ) told her he was ashamed of his behaviour.
She didnt take this well and demanded he talk to her (ive seen the messages) every day there would be a new drama from her, or she would try to make him jealous, even as far as telling him all about the men who were after her on the day of the funeral. He said at this point he couldnt believe how sucked in he got by her and that he could see she was a very unpleasnt person. He told her it was over.
Ive always really looked up to him, and all of this really shook how I saw him but after lots of talking, i decided to try and think of his behaviour as a blip caused by the situation. We felt closer than we probably ever have and I have continued to support him since we all returned.
I have now found out that he shagged her the first opportunity he got. They have both made an utter mockery of his fathers death, his remorse means nothing now. If he meant a word of what he said he wouldnt be able to be in the same room as her let alone fuck her. And she didnt give a flying fuck about any of what was happening and she's got a massive pat on the back for it.
I feel like they are both terribly proud of what theyve done. And im devesated, cant even make sense of it.
Am i wrong in seeing this as awful behaviour? I honestly cant see straight. It feels awful, really dont think i ever knew him at all.