Madgran I don't think it's harsh at all, Nannyone's DD met a nice man after three years. In the five years since her DS has refused to speak to him his mother could have put her foot down very firmly as in you will act politely to my friend in our home. I would have, it's not up to the 22 year old to dictate to his mother how she should live her life, within reason of course ie if she was drinking heavily, neglecting her children for a man etc, that is a different matter.
I was in a similar situation, after my marriage breakup. So I do understand.
Your DS might not have turned out as he has if you didn't have clear boundaries when he was a teenager. It does appear that in Nannyone's DD's life she is being dictated to by her DS and her mother who is trying to spare her GS's feelings. He does need clear boundaries and if he still refuses after five years to speak to his mother's partner, the boundaries aren't strong enough.