My massage therapist is also my good friend... should I tip her?

You should feel weird giving her a tip as she should receiving one from you. I suggest you take her out to lunch once a week.

if she is your good friend then why are you paying her? my mates and i use our skills as favours for each other. i would be extremely offended if one of them offered to pay me let alone tip me

You should totally tip her. Especially because she is your friend. I have a friend who is also a massage therapist and she always protests when I give her a tip but I tell her that it is how she makes her money and if I were going to someone else I would be spending the same amount but I choose to give her my business.

I would treat her to lunch or dinner or take her cakes.

Try talking to her about it. It's her job, so that's how she makes her money. But if she is okay with a nice lunch or girls night out as her payment, I would do that instead because then she also gets to spend non-work time with you.