I get exactly what you saying and wherever I can I do my best to protect his friendship and actively encourage it.
However after being together for 8 years and her carrying for the 4 years, I've said something needs to change.
She knows he can't say much, because she's the kind of woman that would say well you two can't speak anymore, she knows this too and plays up to it and uses it to her advantage wherever possible. So much to the point the she will now invite him to a party and tell him specifically not to bring me, he is only to come, I am not wanted (but please don't tell me, just to tell me I'm clingy)...
I have mentioned to him that I am not looking for him to have an argument with her or anything that could damage his relationship with his mate, I am just asking that instead of him agreeing and then saying to me you can't come but I'm going, that for a change instead say something along the lines, "well I'm sorry but if my partner's not welcome then I won't be attending either, as I want my partner with me'
More than anything I'm just asking him to have my back more, she doesn't like me that's fine, I don't like her much either, however I would never be rude to her face, I was taught to be more respectful that that. But I was also taught that even if you dislike someone's partner you do not deliberately exclude them from social functions as that in itself is disrespectful both people, that you accept that they are a couple.
Lately I've been trying to get him to understand that her unwanted attitudes and comments about our relationship are rude and hurtful and the fact that he also allows it is causing damage to our relationship as I do not feel like a partner to him, that sometimes I feel more like a burden.
I have also explained that I am not trying to come in the way of his friendship, but there are small things he can say or do, that can politely but firmly let her know that she needs to stop with her antics and to stop trying to interfere with our relationship.
He has agreed that her treatment of me is unfair, which is a start and has also agreed that he does need to start having my back more, and has finally started to see things a little more from my perspective.
I have also been getting him to put himself in my shoes and ask how he would feel if it was one of my friends who did it to him, and ask if he'd be hurt, which he admitted he would be and at the very least would expect me to say something.