I agree, but the drummer is quite good, reliable, and a darned fine person. It is not a solution that I would prefer.
So, is the consensus that there is no way to cap him at the mixer? I am going to give the compressor with the ratio set to infinity a try, but I have never tried to use a compressor as a limiter. I was hoping someone may have some guidance in that regard.
Basically he lies about having it set as loud as it will be all night, because, well, he is a lying liar, and he wants to do things the way he wants to do things. Yes, it is a bit like being in a band with a three year oldāwith gray hair and bad knees.
Maybe try leaving the auto gain function active on his channel on the mixer. If he turns up the auto gain will try to keep him at unity. I can only imagine the distortion he is creating by overdriving the gain stage at the mixer. Maybe some better IEMs that seal off better and higher fidelity would help him hear what he is trying to hear.
If I get this right, I think one of the problems is that the keyboard has its own speakers and that turning up the keys also affects the āacousticā mix to the āhouseā as well as the levels of anything going through the mixer. Let me know if thatās true, just for my understanding. The autogain idea is a great one, though.
Yes, maybe that and the compressor set as a limiter will help keep him in his lane.
No speakers on the keyboards. And the bass player is the only one in the band who uses a wedgeāwith almost no keys in it. Everyone else is on IEMs. All of keyboard playerās noise is heading straight to FOH.
We have the same issue with the other guitar player in my band. We both use modelers. I just got my mixer last week so I havenāt used it live yet but I was playing with it last night and did the Auto Gain on my Helix, sure enough, I tried turning it up and the Auto Gain kept it in check. It absolutely will work.
Great news. Thanks for letting me know. Way easier than the āaccidentā with the bucket of water and hammer we were discussing.
Easier but definitely not as much fun 
You need to audition a new Keys player!
Hugh
In case anyone is interested in the resolution, the autogain and brick wall compressor kept him from messing up the FOH. He is the only one still using a wedge, and cranked that thing up enough to cause structural damage. Another problem for another day. Thanks for your help everyone who contributed.
This is the only helpful idea on this thread that Iāve read so far, besides the original idea of using the compressor as a limiter. Good thinking @wildr ā Auto Gain is literally made for the this exact scenario,
Go to Config section, select his keyboard channel
Turn on Auto Gain, leave it on
If he turns up the output on his keyboard and clips the channel on the mixer, it will automatically lower the gain of that channel.
Described in this video
Allen & Heath CQ Series Gain Assistant
Just trying to help where I can. I love the little CQ20B. I have put them through all scenarios and have been able to overcome them all. Now I am patiently waiting for the SQ Rack to ship. Iāve had one on order since the original announcement.