Depends entirely on what she wants the gun for, in my opinion. If she's looking for a handgun for self defense, you can't go wrong with a S&W J Frame. You can always get a 38 Special and look for reduced recoil rounds (or load your own, or buy from me

) for her to practice with and then load it with the "real" self defense rounds for when she would need them.
As for a shotgun, I have to agree with the 870 choice. If she's recoil sensitive, you can always buy her a 20ga. I've actually found that I enjoy a 20ga more when bird hunting or just shooting. They're typically lighter to carry and the recoil is reduced so you can shoot it more without wearing yourself out.
For a rifle, it's a completely utilitarian choice from my standpoint. For plinking, I would definitely recommend a Ruger 10/22. Ruger has a great warranty, the guns are inexpensive to buy and shoot, and there are tons of aftermarket accessories. For medium-large game, I would recommend a bolt action 243 or 270. However, because the bullet weights will be a little lower, shot placement is more important.
There are way too many different rifle configurations to go over here, but I'm sure you see the point.