Yes, I would say charge for "it", depending on what "it" is. If it is a definite technical competency that your firm has spent time & effort devloping, then charge for it.

If it is something that is doable as "cost of sales", then don't.

Depending on the mentoring type (Q&A, Skill Development, or transformational activity), I would charge by the hour or as a project rate.

Good luck!