Would you continue to support this friendship after learning this?

One son who is 19 has this friend he made when he was 14. I was a little uneasy about my son being his friend because he was a loner. A lot of us parents used to gossip a little about him which I feel bad for now because I learned his grandparents who he lived with were beating him and he was being bullied at school. My son just randomly started sitting with him at lunch. He's definitely changed over the years. He's a very sweet kid and is currently in therapy. My son drove him to therapy when his car broke down. They were playing video games in my son's room and I was getting home from. Work and heard them talking. I eavesdropped a little and he told my son something EXTREMELY disturbing. He said he told his therapist and he wanted to tell my son, that year that they became friends he made a plan to shoot up the school and kill himself after. It was not a thought. It was a plan that he wrote about in his journal and that them becoming friends made him change his mind. He at first decided to kill everyone but spare my son, but then decided against it all together. He was telling him because he feels extremely guilty now for even making a plan like that.

That's really scary and disturbing. Especially since school shootings is a huge problem in America. I know he DIDN'T do it but he planned to and the thought I could have had dead children disturbs me. I don't know how I feel about supporting this friendship anymore. He's an adult and can do what he wants but would you talk to your son about rethinking this friendship?