You just slip out the back, Jack.
Make a new plan, Stan.
You don't need to be coy, Roy,
Just get yourself free.
Hop on the bus, Gus,
You don't need to discuss much.
Just drop off the key, Lee,
And get yourself free.
In all seriousness, I’d suggest promising specific regular occasions when you’d be happy to meet your ex as a friend, so that they aren’t left entirely without friendship and without events to look forward to. First Saturday afternoon of each month for the next 12 months, or some such.
I’d also recommend contacting a local autism support group who might be able to check in with your ex periodically to help them recover and move on. Ideally, they’d hang out with your ex right after each time your ex sees you, to help take your ex’s mind off you.