I'm really feeling the urge to ruin my friendship by reporting that a friends ex threatened suicide.
That's a pretty serious matter, why do you want to let the ex's mother know about it?
I have no idea were else I should go to.
Even though it might damage your friendship, it is okay to let someone close to the person know about their suicide. They care about him and can hopefully get them the help he needs.
Maybe I will tell his girlfriend since they are still friends.
The best thing in the situation is the make it known that this person's having these thoughts. They need to get help right away and I recommend letting the right people know about this.