I'm sorry you had to witness that, witnessing a suicide (attempt or otherwise) can mess you up for a long while. I hope you're ok, have you got a professional you can talk to about it?
I have witnessed suicide attempts, two actually. One while I was on holiday walking along the beach during the evening. I saw a lady walking into the water up till about chest deep (thinking she was swimming, an odd time for that but whatever). Then I noticed two others chasing down the beach, yelling something at her I couldn't hear. Didn't stay around too long to see the outcome but I assume they talked her down.
Second was actually in a park near my house, there was a lady looking very distressed on a park bench. She was dripping wet as she had doused herself in petrol. I could tell she was distraught by how much she was shaking. I remember the lighter in her right hand, it clicked a few times but never lit (thank heavens). Everything was getting too much and I wasn't getting anywhere with talking so I called police and ambulance to come help. A nice homeless dude also tried talking her down a bit and hit the petrol can when she wandered off. Police got her and took her to a safe place, I hope she's doing better now.
Just makes me feel really sad, knowing there's others who are going through such mental distress. That our society is so messed up that many are struggling to cope.