Honestly, OP, after reading all of your posts, I get the impression you've been leaning heavily on Cowbag and her DD and it's quite likely Cowbag has deliberately pulled out because she's realised that. Does she really want her DD to have more female friends? Or is it more that she wants her DD to not have any friends who are so reliant on her?
Children cancel on birthday parties at the last minute all the time. They get sick, their parents forget, they misbehave and get pulled from the party as a punishment... any number of things can happen. So, bearing in mind the standard drop out rate and how few close friends your DS has, I think it was always pretty risky to book an activity requiring such a large number of children for it to definitely go ahead.
Given ASD is genetic and just how upset you are about this... have you been assessed yourself? RSD is really difficult to cope with if you don't know that's what you're experiencing, and haven't had any ND-coaching for it.
I'm truly sorry the party is at risk and that you feel so upset about it. But I don't think having a go at the mother of one of only two children your DS gets on with is a smart idea. Cowbag will only distance herself and her DD from you and your DS even further.
I'd accept that Cowbag is unhappy with her DD taking responsibility for your DS's emotional wellbeing, and downgrade her in your head to a classmate rather than a bestie. So, someone who DS enjoys spending time with when he gets a chance, but not someone reliable.
I'd also be careful not to start leaning too heavily on the remaining good friend.
Making friends is hard for most people, let alone those of us who are ND, but why is it so important that DS has loads of friends? Is he unhappy that he doesn't have loads of friends (hence organising a party for 2 x 8 kids) or are you the one who is upset that he isn't fitting into the mould of the popular child, with so many bubbly friends he regularly plays with?
Not all ASD people actually care if they have a big social circle, and are quite content to just have the time and space to pursue their special interests, with or without people. I'd put more energy into facilitating DS's special interests (or finding out what those are). If he makes some more friends along the way as part of that, great. If he doesn't, but he's happy enough as he is, also great.
You say that he's a cheerful lad, but cheerful can mean happy with his own company. And that's not a bad thing.
Put on a brave face, and make the most of what you can salvage. This whole thing has been a tough lesson, but ultimately, it might help you manage better going forward.