Aw, Joe, that sucks.
Even though I still hold firm to my first post that you should be leery about trusting anyone, I still go out of my way to help when I can. I had a similar instance as you described, guy was losing badly to an elite. I was patiently waiting my turn to spawn the same elite so I was just watching him (general common courtesy rule is you don't spawn elites while another player is already engaged). His companion dies and he was on his last 1/4 of health. So I decided to start healing him. He lived and whispered a "thx". I felt pretty good for those short moments.
If there is someone fighting a mob around two or three objectives that you need, stand there and watch or go to the other side of the room to get objectives. Sometimes they may not be on the same planet quest you are and they don't need those objectives but it's still best to wait until they pass on them. Other times that same d00d will reciprocate, meaning he won't ninja objectives that were around me while I was tied up. So it's not all bad, it's just the d-bags seem to stick out more.
This has been my general rule as well. Just stand back and wait, if help looks like it's needed, do so. Usually that same person will help you out in return. I helped a group on kaas once that was having trouble with a boss, I had my upper lvl sniper, we killed her quick. They offered to compensate me, and I told them don't worry about it. Hopefully they will carry that forward, but it was just good gaming, imo.