>>10144344There's a "temporary, don't play with it" fix with is to drill and pin, but that's a pretty fragile fix and will basically mean you can't pose the elbow without breaking it again.
Replacing the broken joint is going to be your best option, which is going to involve drilling out the broken joint and fabricating a new one or finding a donor. Your best bet for creating a replacement might be to cut and sand down a Lego piece since they are made with very strong and dense ABS plastic.
Maybe someone with more experience in figure repair has more ideas, but this is the video I got this idea from.
https://www.youtube.com/watch?v=VI-nLloKJsc 