How should I deal with this situation?

My best friend I've known for 5years... who's more like a brother got together with a woman 3 years ago. For the purpose of this story I'm gonna call my best friend John aged 23 now & his girlfriend Jane aged 29 now. When they 1st got together Jane was in an open relationship with another guy for a couple of years whom she loved, I'm gonna call him Adam for the purposes of the story. Jane was seeing both John & Adam at the same time... both guys knew about one another so there wasn't any secrets. This went on for about 4 maybe 5 months, then Jane broke it off with Adam as whenever she was with him she was thinking of John as she'd fallen in love with him and found that she loved John more than Adam. For the 1st few months John and Jane's relationship was an open one, they decided they only wanted to be with one another so it became a monogamist relationship. Now fast forward a year and a bit... John & Jane are very in love, my best friend is so happy, all is good! We all all go out one Thurs night as its Jane's birthday... I asked Jane if she would marry John at one point, her reply was "Of course" John&Jane went home early that night as Jane was going to Amsterdam with her best friend the next day for a couple of days. John gets a call from Jayne on the Sun, tells him that on previous night she'd had sex with a guy, her because her friend was acting aloof, she went out had reike done which left her feeling bad... so she got drunk and went home and slept with this other guy. Up A week later she tells him that when they were in an open relationship at the beginning she's had sex with Adam 2 times after their breakup. I wasn't told the Adam thing until Jane asked me to have words with john to help him get over Amsterdam but failed to tell me that she'd slept with Adam, i had to hear this from an angry john when i was trying to get him to open up and help him get over the whole Amsterdam thing. N ow 3 years into their relationship, he's still not over it, he's so jealous that Jane isn't allowed to look at anyone hes' depressed has suicidal thoughts, bulimic and micro dosing with a legal analogue of ketamine, admits that this has been caused by Jane. His mother who I'm very close to asked me what's wrong with him as she's really worried about him. and never seen him like this but John asked me not to say a thing about Jane in the past as it would cloud judgement of her. What should i do? Update 3: John since before i knew him experimented with legal highs. At the beginning of their relationship there was 2 occasions that he ended up in a+e due to taking too much and having bad trips, but no one was to tell Jane at the time as john didn't want her to worry or be put off him in anyway so no one did but his mum and i said enough's enough regarding chemicals. John has tried to break up with her once that i know of which resulted in her going into bathroom with scissors but didn't cut herself. U I only found out about everyday drug use recently. And whats worse Jane knows about the bulimia and the drug use and has stated that he looks healthy on ketamine. I've tried to talk to him and help, he just makes excuses for Jane or says he's gonna break up with er but never does. i cant help but feel she wont do anything about the drugs or the other stuff because she has him exactly where she wants him, he won't leave her as he thinks he can't find anyone else, which simply isn't true as practiacally every time we go out any where some woman comments how good looking he is and that they are interested in him, we haven't been out together for a long time.