Is she in private practice? If so, I wouldn't worry about tipping. If she works for someone else, I'd still tip the 20%.
I'm in private practice, and I set my rates for what I need them to be, not what I need them to be if there's a 20% tip on top. Some people tip, some don't, and it's fine with me either way.
When I worked in a spa, it was totally different. I really counted on my tips then because the spa pay was so low.