I think I would lean towards
no since it is you doing the action. To me, that is not imposing beliefs or interfering with free will. My family won't ever come over to my house for dinner because I will not buy meat, cook meat, or store meat in my house. To do any thing other than that would go against my beliefs. They can do whatever they want in their own homes and with their own hands, but I don't want to take part in it. That is both ethical and out of concern for their own health. Sadly, their idea of healthy eating is not even slightly healthy.
Oh, and when I only ate raw, I didn't cook food for anyone either. I was more than happy to prepare anything raw for anyone though
My friends have never asked me to buy their cigarettes, and I do know a lot of people who smoke. I would probably tell them they could come with me and buy their own cigarettes if I was already going out. I don't think I would actively participate any further than that.
A few weeks ago I had a few friends staying the weekend at my house from out of town. They wanted to go get a bottle of alcohol and bring it back to the house. I told them absolutely not. I didn't want to bring that kind of energy into my house and I didn't want to facilitate people I care about doing something that I feel is unhealthy. They thought I was acting ridiculous... I guess that is even a step further than just not buying it for them, huh?