bird shot for training until she gets good and comfortable with the shotgun. The LAST thing you want to do is make her recoil shy.
I might even suggest - if you have a pump-gun that will run it- the aguila mini-shells UNTIL she gets manipulation and technique in place and on point.
Then move her up to bulk 2-3/4" light trap loads. Let her make the move. Household harmony and the firing line are much better if mom doesn't have a should that looks like it was kicked by a horse.
I've been through several shotgun classes for experienced shotgunners and in each one the vast majority of the training was done with bird shot.
I'm not recoil sensitive by any means, and I have soaked up a lot of shotgun recoil - 500+ rounds in a day for several days- but I'm kinda abnormal...
As an example...
I used to shoot 5-10 rounds of skeet (25 birds each) on off-duty days at the Fort Irwin Sportsmans Club skeet complex with 3 1//2" magnum turkey loads in an 870 and not even have a reddened shoulder pocket.
Don't count on the lady being an efficient recoil sponge at the start.