Roommates health kick

So my boyfriend lives with his buddy and his buddy's girlfriend. And I'm here five days out of the week. Today the buddy's girlfriend said that they were going to start eating healthy and she was going to throw out any unhealthy food so if my boyfriend wanted unhealthy food then he needs to hide it in his room. My boyfriend contributes money towards groceries every time she goes shopping but he isn't on this health kick crap that they are doing. He wants me to go grocery shopping for him and let her do their own grocery shopping. I don't want to step on her toes but I don't agree with him giving her money to buy healthy food that he doesn't want. How do I do this civilly and not step on her toes? And how do I know what to buy and what she will buy? HELP PLEASE. I'm trying to avoid conflict