Sensitivity varies a lot from person to person, and some people that are pretty sensitive still won't reach orgasm that way. For others, myself included, it is possible and even pretty damn easy, and honestly fingers alone are plenty. Gentle flicking, grabbing at the whole breast, and circling the araeolae does the trick generally, but it's generally a full body experience for girls so stimulating her elsewhere (neck, torso, legs, etc) helps too. Vibrators don't work quite as well there for me, though they do still feel good. Nipple pumps are a good idea! My partner uses clamps and that also tends to make them pretty sensitive - could be an option depending on if she leans masochist at all.
Have fun!! Sounds like she's in for a good time 😁
HRT too, in some places!