I don't know what priority these points should be in, but I'll say firstly, secondly etc. You can prioritise them as you wish.
My DD has a similar friendship and her friend has issues, so it's really hard, as the child can be nasty, I'm assuming that isn't the case here?
Firstly: you need to text and say I completely understand this was an over sight with the stress you are under. If you could give me more notice that would be great as I like to plan my days, cook in advance, occasionally have plans to visit family. So as not to let you down, the more notice the better.
Secondly: are R & DS friends, does he go round to the other house out of obligation, or does he enjoy the time he spends with R? Basically are they really friends, or does DS tolerate R out of sympathy, R's Mum & your relationship?
Thirdly: Do you have Lego? Maybe for the 'visit' you could plan that the boys have a Lego competition to build something each, the best gets a prize. (Both getting a Kinder Egg or something)
Fourthly: Does R have any other interests over the computer? If so what are they? You could say to R's Mum you've been doing a bit of a reading competition to try and get DC to read more. Could R bring his favourite books, so he could join in and not feel left out? Do they enjoy physical things like baking? Could you make some Easter Cakes together and then get some icing and different sprinkles to top? If the weather is nice you could do a 'Find me' competition. I could knock something up quick and email it over for you to print out. Both DC's have a copy and some sellotape. If this is viable, if you could let me know if you have access to flowers (even daisies/dandelions) a tree? Ask his Mum if she can give you a list of what he likes and what he doesn't? Leading onto 5!
Fifthly: Do any activities trigger R, are his parents aware of anything that is totally no go? Is being at a playpark an issue? Does he have any phobias, sensory issues?
Finally: Ask R's Mum if she is getting adequate support from social services? Let her know that Direct Payments could be a big help to give R 1-2-1 interaction and support. Also ask if she is aware of groups that help? If she's made any friends with other Mums who have children with Aspergers. Be honest ask how is she coping, tell her that she can feel free to vent via email. If anything jumps out you can help with (like services etc) you're more than willing to help.
You understand that it is a challenging time at present. Hopefully if they can get adequate support in place, which will alleviate things.
Whilst you're friends, you shouldn't feel like it is overly impacting you. Your posts read, almost as if you dread your phone ringing because of some crisis. Is she quite isolated? Does she have family to support her? You shouldn't be shouldering this solo, even a counsellor would need to debrief and discuss through things that have been highlighted.
Hopefully there'll be a better picture if you can find time to answer the questions, I can advise you further.
Just because she believes child rearing is a community thing, it doesn't mean you have to agree, it looks like when she says community, she means you. She's going through a rough time, that's obvious, but it shouldn't be impacting you this much.
Look forward to your reply 