My friend is in a really unhealthy relationship with this guy? Help please?

She manipulates him, and he treats her like crap. The other day they were in an argument because he was angry with her for not giving him a hug (she couldn't because this was at church) and she was mad at him for not sitting by her. She was actually CRYING because he didn't sit by her. She's my friend and all, but that is so stupid. She does it all the time. They are always mad at each other because one of them didn't hug each other or kiss each other or sit by the other (and they only see each other at church and that's not the place they should be doing it anyway.) They've managed to be together for almost two years now, but his mom hates my friend, and is always trying to break them up. My friend always manipulates this guy, and he is often mean to her, and they always getting trouble for too much PDA. His mom has broken them up several times but they always manage to see each other and then get in trouble all over again. It never ends. He asked her to marry him a month ago, and she's only 16. I know I don't have a say in what they do/say to each other at all, but how can I convince my friend or maybe talk to her about this and show her that this is not very good? Thanks.