I'm concerned about the mutual friends because I don't want them to be influenced away from me - that's likely just my own doubts in myself. I'm sure they'll understand and stick by me regardless. They're not stupid - they know the person is "very intense". I haven't discussed it with other friends, I think I'm just going to take the "I haven't talked to him in awhile" like another user suggested.
I should also mention I'm a registered nurse with some psychiatric experience, so I partly felt like I could help him. I definitely know all about the benefits & side effects of bipolar medication, a lot of you are quite well educated on the topic! Either way, it is his responsibility to seek help and mine to distance myself if someone is untreated and erratic.
Edit: I should also mention he knows some of my deep secrets, I was worried he would try to use those secrets to try to wreck my relationship with my girlfriend.