I too have one friend who is a JW. She is the only one I still keep in touch with. I had a discussion with her quite some time ago letting her know that I would respect her beliefs as far as not wanting to associate with me but I would never agree with them. I told her that in my opinion a friend is a friend no matter what and that I would be there for her regardless of her religious preferences. I also told her that I expected that any friend of mine would do the same. I let her know I do not agree that friendship is conditional and if she wanted to continue being my friend I would be happy to have her but if she didn't then that would be her decision.
She still calls me and we do not discuss religion. We have periods of time where we don't communicate but eventually I or herself will call and check in. I actually think she likes the fact that she can talk to me about the drama she experiences with people in that religion and with elders knowing I am not going to judge her or tell her she needs to keep it to herself. I'm kind of a sounding board for her and every once in awhile I slip something into the conversation that shows how wrong they really are when it comes to the Bible without being to aggressive.
The other day I was talking to my daughter about how the JWs don't believe you should associate with people outside their religion. She knew it but didn't really believe her dad enforced it. I told her that when she is at our house we are tolerant of people with different beliefs and we don't believe God is going to destroy anyone who wants to worship him but that her dad will not allow her to associate with her friends at school. She argued and said he lets her talk on the phone with them. I said wait until you ask if they can come to your house or if you can go to theirs and left it at that.
Yesterday she told me her friend from school asked if she could go to the movies this Friday, which is her dad's day and he said no. I asked her why he said no. She said because her friend is worldly. I laughed and asked her if that made any sense when Jesus was accepting of everyone who wanted to worship God regardless of their religion or profession. She said no. She got the point though that JWs are completely intolerant and relationships are conditional. She knows now that one day they will expect her not to associate with her own mother, brother and step dad. She knows that's how rediculous they are...
Sorry you are losing a friend over it.