That is an awful situation to be in and i can completely relate. I know it must be hard, trying to support his girlfriend who is devastated and feeling guilty as well as supporting your friend who clearly is having some serious problems and issues to deal with. Seeing as you are friends with his girlfriend, i think you two might be of some comfort to each other. You both are trying to deal with someone close to you who is contemplating suicide and making small attempts on several occasions. It must be really hard to see him behave in this way. As well as trying to support his girlfriend, it sounds like he really needs the support of those around him too. He clearly needs help- although his overdoses haven't been large enough to kill him, he is still playing with the idea in his head and that proves he isn't in the right frame of mind. Let him know that you are there if he needs you, but also give him space if that is what he wants. As sad as this is for me to say this, at the end of the day there isn't a massive amount you can do. He needs professional help.
At the same time, i'd also like to say i think you and his girlfriend could do with some external support. Have you thought about talking to a counsellor? You have been given a huge amount to deal with, and it sounds like there are some feelings of guilt lingering around that shouldn't be there. You and his girlfriend have done nothing wrong- he is mentally ill and there is nothing you can or could have done to prevent it happening.
I know just how much of a tough time you are going through- i myself came home one day a few months ago to find my mum had taken a massive, massive overdose and had locked the house from the inside so no one could get in to find her in time. I strangely came home early from college that day, couldn't get in so called the police and thats when she was found. I knew she had been suffering depression but it is still an almighty shock. She is still alive now but in the same mindset and has made other attempts, all of which have now severely damaged her health. Its a really awful situation to be in, but if you ever want someone to talk to who knows what you are going through from a friend/ family POV of a suicidal person then just send a PM (this was the whole reason i decided not to post anonymously)
I hope things work out, and take care of yourself.
Lou
xxx