As detached as she is comfortable being in a friends with benefits relationship.

Don’t forget the friends part and focus solely on the “benefits.” It’s not strangers with benefits, after all.

Regardless of what happens to the second part, if all parties keep the first part strong, everyone can (1) walk away feeling good about themselves should it end, (2) continue as long as it’s working for all involved, or (3) renegotiate the type of relationship they’d like to have.

I won’t assume [a] the involved parties are a man and a woman, [b] monogomy, [c] gender essentialism, and/or [c] heterosexuality. The situation could be two or more women, a woman with one or more gender-neutral or gender-fluid individuals, or some other configuration I haven’t thought of.  That’s why my answer is as open-ended as it is.