Extremely long nails easily fall into the "it's all relative" category: What one woman considers long might be nothing compared to another woman's idea of truly lengthy nails.
There are nails that are long, yet still considered acceptable to sport in professional environments.
You should be able to express your likes, dislikes..you would like to explore and try. This is probably something you'd be more comfortable bringing up to a woman with whom you are in a relationship, as opposed to a casual sex partner. If she asks why (because she's never done it or heard about it) then explain you are curious and would like to try. Otherwise consider rinsing with a bulb syringe and plain warm water, or a plain water enema first. it's best if there is some barrier used like a finger cot, condom gloves. Without providing too many details (which wood be rude) she tried it.
Tell her what you said here:"I've read that prostate massage can be both medically beneficial and extremely sexually stimulating. He was slightly hesitant to bring the subject up but as soon as he began to mention it, I was totally on-board. This will prevent any possible scratches from her finger nails. Due to her extreme INexperience the process was only fabulous!
For example, extremely long nails simply don't pass muster in the workplace, and they can make even the most basic daily activities challenging.