Best Friend issues

Shopping at the mall for clothes- I pay

Eating out - I pay

Driving places - I pay

Shoe shopping - I pay 

Pretty much whenever we eat drink shop or hang I foot the bill. 

I have NEVER asked her or even once complained because she lives at home with her parent with her son and fiancée. They both have good jobs and are 20+ 

So she's an adult with an adult life. Lately I've asked to hang out more since my wedding is a few weeks away and I have to pay for her hair, make up, dress, etc... and I wanted to get a fitting and see what hairstyle she wants. She's been MIA and PNLY contacted me to buy her dog.. which I did.. 

She's also asked me NOT to tell he fiancée she's cheating on him when she openly told me over lunch she's been with this other guy for a while. That was a step to far. So I said sure, but you need to tel him because it's not fair to him and your going to marry him and you have a kid with him. She won't leave him or anything she keeps it up. Well I said my peace about it to her after 3 months of him not knowing and said she needed to tel him it wasn't right. 

Now she's completely stopped talking to me. Wouldn't come to my sons party and won't acknowledge my messages. She'll read them but not reply.

So my question is would the adult thing to do would be to write out a message without any blame or anything negative and say hey this isn't okay I feel taken advantage of or should I completely freak the hell out? 

Sorry for the long thread lol. I'm trying to be an adult I've just dropped some serious cash on our relationship and shes been influential in my life and my families and now she's just acting like I never existed.