Positrack, good going. Just found this thread, very good advice here.
A couple of things. First, go ahead and get a safe. For quick access, consider a GunVault for long guns. It would be safer not only now, but certainly if you may have young visitors, like her niece.
Second, think about finding a youth model 870 20 ga. Try some pawn shops, buy it used. Prob cost around $100, will fit her better, lighter, less recoil, and ultimately, with buckshot, is a very powerful SD gun.
Corneredcat is a great resource, as is "The Best Defense" show. As was said earlier, ask her to watch it as a favor to you. Stress that it is NOT a "gun" show. A lot of the content has to do with awareness and tactics not involving firearms. My wife actually said she learned a lot and was not turned off by Janich and Pincus's style. It is not condescending or dripping with testosterone like she was afraid it would be. I asked her to watch it as a favor to me, 1/2 hour a week.... it is in reruns right now, and I don't know if you will be able to find a way to start at the beginning, but would really encourage you to try. It has a very well thought out progression in the topics presented.
If she gets to the point where she is considering trying the handgun, get her a copy of Vicki Farnam's book "Women Learning to Shoot: A Guide for Law Enforcement Officers". It is written for women, by women. All the basics, plus a little more attention to techniques specific to problems unique to women shooters.
Hope things continue to go well. You have a big advantage in that you have both approached this as a difference between you, but not a deal breaker.
Steve