"suck it up, buttercup..."
That's pretty much it. In service rifle you're limited on modifications so you adjust the position to the rifle. I ran into this quite a bit coaching for the NG. You get all types and shapes. Being in charge of the Marksmanship Unit also made me OIC of SARTS (Small Arms Readiness Training Section) or qualification of the whole state, be sides the Guard we got the Reserves, Navy, and Militia. All shapes and sizes.
I had a couple ladies who were quite small, one with short arms to boot. They kept saying they wanted shorter stocks. Ok I made some short M14 stocks, Be careful what you ask for, those short stocks beat the crap out of them. Remember this was M14s, not ARs, Recoil is a bit different.
After getting wacked pretty good they slid their faces back on the stock. Didn't get hit, and shot better. I say if they can do that with the short stock why can't they do it with the standard M14 stocks. Went back to the regular M14s and no problems.
The one mentioned (with the short arms) had a good cross legged setting position, didn't work when she started pulling the trigger. I moved her to the open legged position and she ended up being the best lady setting rapid shooter I've seen, cleaned a lot of 200 yard rapid fire targets.
She was quite small as I said, and petite, doubt she weighed 95 lbs. She had to put bullets in her pocket to make weight for the guards height/weight standards. But when I took her to Hawaii or 29 Palms I won a lot of bets against marines betting on her rapid fire scores against big tough marines.
Anyway, its not about the size of the rifle, or the shooter, Service rifles aren't shotguns, they can't be modified. So you modify the position.
Hard to say what to do on the internet, you need to watch the shooter a bit before you can make adjustments. Need to find a coach who knows how to work with such problems. As Gary Anderson says, "there is no hopeless shooters".
As to the OP, you name indicates you live in Alaska, (though not stated). If that's the case, hit up the Alaska Rifle Club and get a hold of a Ron Flashhacker, tell him I sent you, have him work with you're shooter, He'll line you up.