Oh boy...
I've been that person who was in soo deep with a clearly manipulative person, once... :c
Sorry to say, but it indeed isn't easy to convince a friend that they're being manipulated by someone. I know, because I also wouldn't believe my friends when they told me that my oh-so-sweet girlfriend was actually being a huge c8nt to all of them. Only after about a year I found out that I had lost almost all of my friends because "my little sweatheart" (Wich I later found out was indeed a huge c8nt) forced me to spend all of my free time with her, and refused to let me see my friends, because she didn't like them...
However, if the situation of your friend is similair to what mine was, I don't think that you're entirely powerless.
Manipulation by definition is a negative thing. (Else it's called "influencing" IIRC) And, if your friend is indeed being manipulated he should deffinetly experience those things negatively.
What you could try is approaching your friend and trying to get to speak to him
alone, carefully bringing those negative things to attention in your conversation, with the intent of maybe giving your friend a push in the right direction of considering
for himself wether or not the downs of his "(girl)friend" are worth the ups.
It's really important to not come over as negative about this person manipulating your friend. Because, if your friend is really "deep into it", then it's likely that they're just going to choose the manipulator over you. Especially if you come over as "just another one of those a-holes trying to criticise their "(girl)friend"".
My advice is, try to get your friend to vent about how their manipulator has effected them in a negative way, and try to give them a little nudge of truth, saying something among the lines of "Oh, wow. That's not a very nice thing to do. Maybe you should talk to him/her about that?". Just little nudges towards the fact this person is bad for him.
Now I cannot stress this enough:
Let your friend think for him/herself! Do not try to shove it down their troath!
Like I said, if your friend is being manipulated, he likely thinks of his manipulator as a "friend" or someone benificial to them. Immediatly conflicting that belief will just result to your friend running back to his manipulator, who will obviously just keep your friend reeled in.
You might not solve the problem straight away, but it may accelerate the process of your friend finding out that this person is bad for them.
I really hope your friend gets outta this one!