Look girl I get that you need any kind of advice, but this is so beyond internet help. This thing that happened ( and I really hope it didn't honestly, I really hope it's just a creative sad story ) it's over the capability of a nineteen years old comprehension also. He should not had do it and he shouldn't have told you, bcs this sets you in a position of complicity. In my country, this is enough to go to jail, it wouldn't matter if Matt was dying already, we don't do euthanasia at the dorm just bcs we feeling to do it, in any case. Now that this is set I would tell you to advice your boyfriend to talk to a parent or an adult very trusted figure.

I know that you are 19 an think that you are adults, but you are not, you're just above the line and one year or two over the 17 doesn't make you an adult ( prevsly dumb decisions have demonstrated enough ). You have two choices: get involved ( wich I don't recommend ) or forget about it. You can also run the fuck off him for what I know, and nobody could judge you. After that, if you are feeling like you can't get over it. Go to a therapist. Tell the story or tell a similar story without involving Matt or Jake, in a way that your therapist can get you to grieve and overcome this properly. Also, telling you this story, was selfish asf.