I had a friend like this when I was a single mom. They very well knew that I couldn't have done things on my own financially, but they enjoyed our company and paid our way on several things. I knew because of the friendship I wasn't taking advantage and she wasn't expecting me to pay her back. I'd have them over for dinner, babysit when they needed it and was just myself. They never looked 'down' on me or were upset because I couldn't afford it, money was not important to them, they liked to have fun. I say if she wants to pay, then quit feeling inferior just because they have the money and you don't. Money is not important to her, having fun is. So do what you can, you can be honest with her about your feelings, but don't put it on her, it's a pride issue. She's not trying to 'one-up' you and flaunt her money. You're not suggesting these things, she is.
I also have a single friend who has been in our family for over 20 years, she lavishes my kids with electronics, toys & whatever when she wants because it makes her happy. She gives us her 'left-over' iPhones, GPS, televisions etc, never expects a thing, she just likes all the newest stuff and blesses us to get rid of the old. We never expect it and she knows it.
Personally, having been in the situation to pay someone elses way after all these years, I was more blessed to pay it forward, I think it's wonderful and generous of them.
You don't have to do everything they suggest to pay for, but if she wants to go for a massage or whatever, go, enjoy and pay it forward.