I've been around the bruins up here, thankfully never *too* close to the brown ones. My take on it is this:
--She is not too small to use a 12 ga to shoot skeet, but she is probably too small to use a slug gun loaded with Brenneke hardcasts. The recoil from a slug gun is quite intense even for a full grown man, and if your rounds aren't recoiling much they're probably not powerful enough for bear. Don't even think about using buckshot on a brown or polar bear in DLP, it's like shooting the bear multiple times with a .32 ACP firing roundball. The recoil will be very bad in a pinch and contrary to the myths she WILL feel it and it WILL effect her shooting. The mental effects of having a firearm that hurts you are MAGNIFIED in times of crisis, not minimized. This is easy to test by simply doing some jogging, pushups and getting your heart rate up then grabbing the big iron that scares you and seeing how well you're shooting with it. Plus you won't have ear protection which means the slug gun or cut down big bore levergun will be damaging your ears as well as your shoulder. For a woman less than half my size I think that's madness.
--The big boomer leverguns will also probably have too much recoil, at least when loaded with the proper medicine. You can have her try a .35 Remington levergun with 200 grain slugs. Historically these were fine bear medicine and brought home some enormous brown bear. Judge Folta, the bear killing judge, used the .35 in an old automatic for many decades. Also consider a Winchester 92 clone in .44 magnum. I've found recoil to be mild but it hits hard esp. out of a long rifle. The caveat is not all of the 92's and other .44 mag leverguns will cycle the max OAL FN hardcasts, so you may have to make adjustments.
--She should consider getting a bolt action rifle with iron sights, and will need to get some hands-on training and sizing to see what's going to fit her extra small frame. I believe she would be able to handle a properly balanced bolt action rifle in the .30'06 class. Firing heavy bullets that should be sufficient. If she can manage more recoil, consider moving up to light magnums. I very much doubt she'll be comfortable with .338 WM but she can at least try it.
--Whatever she ends up with, she has to have some basic training with it and she has to be as comfortable as possible with it. This consideration trumps all others! A rifle she's scared of and uncomfortable with is dead weight on the trail and useless against a charging bear. She'll flinch, hestitate or pull off. So if she's most comfortable with a .35 Rem. levergun and hates the bolt actions, go for that.