I feel your pain man. Same thing actually happened to me a couple months ago. Lucky for you I documented what I did. I'll explain what I did but ultimately its your choice if you want to try the same (disclaimer
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If its a clean break, which it seems to be, try Gorilla Glue (Home Depot).
You will have to open up the thigh (two screws), remove the bottom half of the leg (knee joint attached) and carefully apply a few layers of glue to the break. I used three layers, inside and outside edge. Don't rush it. I did mine over 3 days to allow adequate drying time between applications. I also smoothed the glued areas between applications.
Here's the tricky part. Because the glue adds thickness to the inside edge of the knee joint, you need to scrape the area marked in my picture (quite significantly). Again, take your time. Scrape a little and try to attach the joint. If its doesnt go, scrape a little more and try again. This might take a while to get to the correct thickness. Whatever you do, DO NOT FORCE IT! You will need a very fine narrow blade exacto knife to scrape the glued areas smooth and to squeeze into that small hole.
Since your joint hasnt even snapped yet, I think you have a great chance to fix this just with a few layers of glue and some scraping. Of course, once fixed, avoid moving the joint just in case.
I attached my joint in the position I needed it posed before I screwed it back together.
Good Luck!