is it maybe better for all if you were to go to their home to visit and the boys play?
My DS is always calmer, happier in his own space...he doesn't really play along with other kids but can play alongside iykwim...it makes a big difference if her DS is calm and able to cope with the environment, etc
Maybe you and DS going to their house would be better for you all...
I understand it's difficult and you want to stay friends and find ways to make things easier for you all.
I think your friend sounds quite overwhelmed. There is many a feeling of bereavement for any parent realising there child is different, it's not so easy for them and your hopes and dreams of parenthood and who your child will be is compromised by them having a condition(s) which mean so many issues and difficulties you had no thought of beforehand. I think your friend is very likely to be feeling marginalised, isolated, lonely and far more worried and concerned all the time....she has a great deal on her plate to deal with. I can see she is maybe clinging to you and needing your support, esp if there are few other people around her.
I think she will have walked off and this is more a clear sign that she is struggling and overwhelmed...she also felt she could do this with you and be this honest...she has showed you how she is really feeling perhaps.
I think it's a huge red flag that she needs help...sooner rather than later tbh.
You say yourself how uptight and upset you are with her DS when round at your house and having tantrums, etc....she is dealing all the time with this...imagine being in her shoes....it's so very hard 24/7 for anyone.
Maybe you could do some research on finding groups, social outlets, carers groups and things...your friend really needs support and it would be good to be able to find this from others with knowledge and understanding.
Is she getting any support through specialists in behaviour management, handling and teaching for her DS at all?....she needs resources and support to do these things. Working routines, home environment, practical things which help her to help her DS, etc
Maybe support her to try and seek/secure some of this from any meetings or appointments and go with her to group things if she needs a hand holding to go there....maybe she needs an advocate to help her with this...NAS, etc are very helpful with many many things....
It must be awful at home if her DH isn't supportive and maybe in a bit of denial about his son's issues....more common than you may think...she is stuck dealing with this loneliness too it sounds like.
I hope any of this has been of some help...I really hope you can find ways of staying friends and working to support your friend...she will work her way through all this...it would be so much better if she had a friend do help her do this though