I cannot see involving bf in this unless he was standing right there and said something in conjunction with OP, or OP could have just said something herself.
Again, while I agree with you, Pea-n-Me in theory on about 90%....
Having BTDT myself, I feel COMPELLED, out of consideration to the OP, to say that I can not disagree with this one particular comment any more strongly.
It is not a matter of a woman not being able to fight her own battles.
OP, you should never, EVER, feel that you have to go-to-battle with your SO's mother. to defend your own reasonable healthy personal boundaries.
There should be NO battle.
Your BF should NEVER, EVER, expect this of you.
I simply can not express this strongly enough.
This is his mother. Has been his mother. And will ALWAYS be his mother forever and ever.... If he thinks you should have to either acquiesce or do-battle to protect yourself from her... I can PROMISE you that this will be an ongoing battle that you will be fighting for many, many, years to come.
When I read this:
finally his mom said "I already heard you, you've made your point and now I'm going to tell you to shut up"
This is what SCARES me.
I am talking horror-movie, Mommie Dearest, scary.
This woman is being purposeful, brazen, and unstoppable.
That, and the fact that you were 'warned' by somebody who knows this guy and his family.
And this, I think ,says it all....
hehehe, totally! He is still very much "her little boy." Drives me crazy! For Pete's sake, she still buys him clothes, I really hate that. Honestly, he's 31 years old, I think he can handle buying his own clothing. .... I don't think she's cut the umbilical cord.
Also, note, that it is VERY common for this kind of thing to only come out with a MIL at the point where the son's relationship with another woman becomes serious. ( engagement, moving in, marriage, etc.....)
It is VERY common for a young woman to see a huge change and to become blindsided at this particular time.
You need to deal with situation with your BF ONLY.....
It is HIS mother, HIS home, HIS significant other... And he needs to be able to put on his big-boy pants and deal with it. As in, yesterday.
You tried to mention that you thought that your BF could be assertive with his mother....
But, NEWSFLASH..... Wake up call.
What you have been describing is, quite obviously, NOT assertive.
You need to see that he can cut that umbilical cord right now, himself, personally...
Which, again, has NOTHING to do with you, yourself, speaking to/confronting his mother.