She's fucked. She'll need a steel rod in it since it's a spiral fracture, which is the worst brake you can get.
Pro arm wrestlers (most) stay squared up. Some will go into dangerous positions to try and win, like guys that have been doing it for 15 years or more. The moment you start looking away from your hand and turn away from it, this shit happens. Although in this case that didn't happen and he was just obviously too strong.
It's a lot safer if you know what you're doing than doing shit like this. I've armwrestled a little (not super serious) with my wife and she's got a huge interest in it too and she's pretty good.
Almost all the break videos I've seen on kitchen tables is because one person is completely facing away from their arm and your just asking for a spiral brake.
There's a few in here, and the thumbnail illustrates what I mean with a bad position. It doesn't happen all the time, and it could be just brute strength even if both are in "safe" positions. You need to try and have your hand as close as you can to your shoulder and try to not look away from it. You don't really use your arm to pin, it's a body movement and your arm is more or less a hook. Armwrestling is a bicep/shoulder/lat/tricep sport.