Here's a story; a couple of weeks ago I had to call the ER because my girlfriend, who has borderline personality disorder, had swallowed her antidepressant + sleeping pills after locking herself up in the bathroom.
There was puke everywhere and I was really scared, worst part being I was locked outside, and tried to knock down the door but couldn't in time to stop her from swallowing the pills, so I kind of know how it feels to be powerless when facing a situation like that, although in your case, it's worst, because there's nothing you can do; all he has to do is go offline, and there no way to contact him; at least I could talk to her through the door etc... Anyways, so the ER comes, I show them the pill and we drive to the hospital.
On the way there, they told me that the she hadn't swallowed enough pills for them to harm her, and as I sit by her, in her pill induced trip she tells me: "I'm going to be alright, don't worry, I counted them (the pills)". So i sit here like what the fuck why would she do that to herself?? But during the night at the hospital, I realized that if someone is willing to put themselves through serious shit like that, may it be swallowing just enough pills for it to be scary or messaging your online friend how badly you want to die and listing the shit you may or may not have taken, there's a problem.
They're calling out for help, and if you don't respond well, and just say like "okay dude, whatever, you're over doing it, you want attention", then you're putting your friend in danger. After that experience at the hospital, my girlfriend and I always talk about it, what led to it, why it happened, got all personal with her initimate life and have gotten alot closer, and I think that's what she wanted, and that's probably what Matt wants aswell.
Talk to him, ask him what's wrong, take him seriously. If he's prepared to go as far as to message his friend that he's about to take 600mg of pills, he needs help, and he wants yours, so be there for him