John,
Best bet is to let her decide what she can handle. First step, if she is not already a reasonably accomplished shooter, is to get some basic lessons to:
1) determine master eye and 'handedness' and make sure they are on the same side
2) determine proper fit requirements for her (mainly length of pull for the stock)
3) impart proper form in mounting and gunhandling
4) practice proper form until it is learned reasonably well (note: no shooting involved so far)
Then locate a variety of guns she can have access to and shoot, starting with lower gauge guns with light loads and working up to larger gauges. Note that proper fit and proper form go a long way to contributing to a shooter's ability to handle recoil, and there is no substitute for experience. Also note that perceived recoil is heavier from lighter weight guns than from heavier ones, all else being equal. In other words, don't start with a lightweight single shot loaded with magnums.
I think a .410 could work for you if that is what she prefers. With buckshot or slug loads it is nothing to sneeze at. But it is still necessary to hit what you shoot at for any load to be effective, and it is quite possible to miss with a shotgun at close range. A shooter needs to be completely comfortable with what they are shooting, and to have enough experience with it that use/manipulation is pretty much instinctive. That can only come with regular practice. She should be willing if not eager to practice with whatever HD gun you both decide on, or else it is less likely to be used effectively in the event it is necessary. If it is uncomfortable to shoot she will not want to practice with it and her skill levels will not be what they need to be.
Larger gauges are likely to be more effective to some degree, though given good hits with a .410 the arguments are likely moot. I'd not worry about it too much if she wants a .410.
Good luck,
lpl/nc
