That's the problem. No one in that situation (as you retold it) seems to have their "head inside their behind"; you described the most positive, yet realistic situation that a trans person will find when coming out to his parents; they're not bigoted assholes, they're not abusive, they don't seem fire-and-brimstone zealots, they're just human beings who have been told that their daughter had been suffering for years with a life-changing secret, and now they have to cope with the guilt, the ignorance, and get used to the idea that the person they loved wasn't what they thought; that would be hard for anyone, and "I'm sad about losing you"(assuming she said it on a spiritual sense, rather than in a "because you need to leave home now" sense) is only the most honest answer he could get, because if they love him, it's true. And if they love him, they will accept his choice and eventually understand it.
And this means they will say some "BS" as you put it, as they adjust. It won't be frequent, hopefully, but it will happen. Live with it; it's not about you and your moral outrage (because yes, it is very much moral; you're judging them harshly based on something that doesn't have the same emotional charge to them it has to you), it's about being a good friend, and not making an already hard and uncomfortable situation worse for him and his parents.
If you do as you say and "wipe the floor with his mom", say "f*** I'm doing this" and "I have the master balls to interrupt a private conversation and make a scandal", you're not helping anyone but yourself, because if what you said before is a sample of the kind of things you might find find offensive (on other people's behalf, I might add), the only person with a happier mind will be you because you're so "strong" and "brave" and "have the balls"; that's called "A bully", regardless of how good your cause might be.
Do it if he's actually coming under attack, but it seems your definition of "trans abuse" needs some adjustment to consider sympathy and understanding for the other persons (things are not so clear as they look, usually), and you definitely need to be more diplomatic, in general; if you do what you said you would, you would be talking on behalf of your friend, who is representing all of trans people.
That would make them, not just you, look bad, and they don't need that, okay? he probably deals with enough crap already, he doesn't need to play negotiator between you and his parents, or alienate his parents on your behalf, or alienate you on their behalf.
So, please, take some time to think about this, and don't make things harder for him, he needs both his friend and his parents there.