When we were trying to do the same for our girlfriend we ultimately had luck using tongue and vibrator at the same time…Either one on each nip or if you can pull it off both on the same one. Try to alternate and give the areas around some attention too if she likes that. Can be kinda like a clit where too much for too long can get kinda…overwhelming in a non orgasmic way. Girlfriend likes sucking and lite biting on them so that helped too. She likes lighter touch with the wand while for myself i occasionally like to press the wand up against the underboob and experiment with speed until the wavelength is just right to stimulate out to the nipple/areola. The indirectness of it can be a nice ‘surprise’ and keep things free ‘on top’ for whatever implement you wanna use.

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 😁