@wizzywig
I feel for your friend op, I was in a similar situation (my kids are older now though). You want to have the normal parent experience and you want so badly for your kids to be treated well. It doesn't always happen. Unfortunately, she is likely to find that this is the first in a long list of ended friendships. Maybe the best is that you remain a good friend to her?
I think we all feel for the OP's friend, but it's hard to tell from the OP's posts whether anyone is being 'unreasonable' at all here
she says she nearly lost her friend in the past because the friend found the comparisons between her son and the OP's son very difficult, which again, I think most people could absolutely sympathise with but the friend needs to decide whether it's important for her to maintain the friendship, in which case she will need to find a way (maybe via therapy) of dealing with her own understandably complex feelings. and/or see the OP separately.
The OP can't expect a three year old to continually practice compassion when he's still too young to understand developmental delays -- for him, presumably, the other child is just experienced as a largely silent, still presence, if the OP says he doesn't speak or move much and largely stands in the middle of the room when they're together. He might even find it threatening. In which case the OP can work on explaining things to him.
But I do think that the OP can't put the responsibility for maintaining the friendship on her three-year-old's shoulders. The adult friendship is a separate thing to the boys playing together.
I really feel for the friend. DS had a classmate badly affected by Foetal Alcohol Syndrome and significant speech delay, and I know his mother struggled to try to maintain some kind of social life for him, because if you had him at your house for the playdate, he would run amuk smashing things. DS, understandably, after having his bedroom curtains torn down and his Lego smashed, wasn't keen on seeing him. We compromised by meeting at the park with other children.