It would likely work for what you are planning, but keep in mind you would be getting his stereo mix of the drums, and not a nice set of 4 or 5 or more drum tracks you could mix in.
It's not a terrible way to get drums though, and [with an AI] you'd only get more, usable tracks assuming he's actually got a bunch of mics to set up and either knows what to do or takes time to learn. (I've recorded some friends with a cheapo electric kit with only L/R out and it's been a good place to start.) But, if you get stereo tracks, you'll probably have to do some EQ, compression and panning, and hope he's not some mad cymbal banger who's never recorded himself before. With only stereo, or a couple mics, the drummer has to usually behave a little differently than live. And then, there's the room where he'll be recording...
I'd give it a try while you're deciding, especially if he's got mics and used to micing his kit (maybe why he has the mixer?). But, for your purposes, it also might be as easy, or even easier, to plug a digital recorder with line-in capability into the mixer and just take the stereo analog files and drop them into your project, and let him drum away while listening to your tracks. Just a thought.