I inwardly seethe if anyone drops around with no notice and uninvited.
I like to decide how I/we spend our time ourselves not have it imposed by others whims.
OP lots of people here are reading this and feeling the pain of rejection of their own child in your posts and therefore finding this very hard and responding, understandably, very emtionally and defensively.
It's reasonable to expect you to support your friend, and it sounds like you have been, and there are lots of good suggestions on here for how you can continue to maintain the friendship with a more structured arrangement.
It's unreasonable of her to use you as a crutch because she's not coping. It's unreasonable of her to drop in, send the boys off to play unsupervised and leave you to deal with meltdowns in public.
Her son is her primary concern and the centre of her life.
This friendship is not your primary concern or the centre of your life. You have your own son, your family and other friends.
That's the tough truth.
A friend will continue to keep in touch, arrange get togeters with and without the children, but does not have to be prepared to make big sacrifices in others areas of their life for that friend. Other things will sometimes be more important. Most friendships are like this, despite the MN calls of 'terrible friend' if you don't devote yourself to someone's troubles totally.
Some friendships, often very longstanding deep friendships, are ones where you can call each other anytime day or night and everything else will be dropped, and where ongoing selfless support would be expected, but even these come under strain when the support starts to drain one party, however much they love thier friend.
I think Quint has a good positive friendship model she's described on here with her friend and sons. They are friends and they've found a way to make this work that suits them both and all their children. It sounds like at the moment the arrangement is just suiting your friend and not you.