I have a friend who's a prostitute. I met her in a parlour (living room), was not punting her that day and have never punted her since, I don't fancy her (she is pretty, but not "my type") but we get along really well (hence our becoming friends.) I'm a bit older than her (no surprise there) and I suspect I've become a bit of a "source of wisdom/sounding board" figure in her life which on her part may be what drives the friendship. Anyway, that's my guess.
The friendship started with a long, random chat in said waiting room one day. She later called me and suggested a coffee (even though I'd given her my number because I'd enjoyed our conversation, I did not expect to ever hear from her again) and we now chat pretty often (weekly, at least) and meet up for lunch/coffee about as frequently as I meet with any other friend. There is no "sex talk" between us...I don't think I am her type physically any more than she's mine...but there is a definite personality dynamic and it seems we're both happy to roll with that. I wouldn't (obviously) say she is my best friend or even a close friend, but she is absolutely as much a friend as the majority of people in my circle.
So my conclusions with regard to the thread title are 1. Yes, you can be friends with a WG...but 2. (and this is the key), NOT if you have a professional relationship with her (i.e. you punt her). She has to be a friend first. Her job has to be incidental.
She (my WG friend) does, by the way, provide an interesting window into her job (through our conversations) and less deliberately, into the mental journey a girl in this job goes through. She doesn't discuss this explicitly (we don't deliberately talk about her work) but over time I have seen her "change" as a result of her immersion into the industry.
Another interesting aside is that when she has friends who are considering entering the industry (amazingly, it seems girls do share this stuff with their mates so it does happen), she often ropes me in to have coffee with them so they can get honest answers to their questions about what to expect from the job.