reduce the load - always a good start.

Agree with Treeman's load advice. A 24 gram shot charge at around 1000fps being about as inoffensive as it gets. In addition to that tweaking the length of pull if possible to get it fitting her nicely and coaching on technique. Getting the gun mounted firmly with a proper check weld while in a good 'boxing' stance and using the push pull technique makes just about any 12G load manageable for smaller and lighter people.

My daughter got completely comfortable shooting my silly fast 00 buck and 560gr slug loads out of a lightweight Mossberg 930 once she got the mount, stance and 'push pull' / 'stretching the action' thing right when she was a 50ish kg teen.   

Reduced load would be a great start, thanks! (It now makes sense why the gun shop has such light loads!)

Where can I get proper info on fitting a stock? I know about LOP, but what about the cant and cast of the stock, drop at heel etc - how does one determine that? (I have a good piece of wallnut and would like to make her a proper fitting stock.) 

From : https://www.shotgunlife.com/briefs/how-to-measure-yourself-for-a-shotgun-fit.html

"Start by standing upright and relaxed, ensuring your body is in its natural shooting position. Measure the length from the crook of your elbow to the first crease of your trigger finger. This measurement will give you the ideal length for your shotgun’s stock.

Next, measure the distance from your cheekbone to the center of your eye. This measurement will determine the appropriate height for your shotgun’s comb.

Finally, measure the distance from the center of your eye to the palm of your supporting hand. This measurement will help determine the proper cast for the shotgun’s stock."

https://www.newhuntersguide.com/2022/11/12/how-to-fit-a-shotgun-to-you-length-of-pull-comb-height-cast/

https://gatdaily.com/articles/shotgun-fit-understanding-stocks/

Loads with 21gr of shot are still very effective and I have used them to shoot rockies. 

Would you all please check the following, especially if you have well fitting shotguns: 

- height from eye pupil to cheek bone equals drop-to-comb equals length of first digit of thumb

- distance of drop-to-heel equals distance over shooter's knuckles 

- width of eye equals cast of stock

These might not be correct, but at least we get a start on using regular body features to obtain shotgun fitting measurements. Please make adjustments to this theory?