The golden rule is to avoid putting yourself in danger to help someone else.
I can tell you my horror story about an insta-buddy on a tropical dive trip, our lack of communication, and how we ending up splitting up mid-dive.
This was dive #41 for me, so I was quite inexperienced (but had done just enough diving to think I knew what I was doing). My buddy, also a newer diver, had a new Go Pro, and his focus was on the video camera, at the expense of most everything else (including buoyancy, kicking, and any kind of communication with me).
It was a deep dive site which began as a swim through a short tunnel, emerging on a vertical wall. The exit from the swim through was around 105 ft deep, and we had agreed that would be our max depth. I followed my buddy through, and along the way he kicked up a good amount of sand. We emerged onto the wall, and he was already below me. I stayed at our agreed depth and swam along above him. Again fixated on the Go Pro, he started descending. I tried to get his attention, but I could not. He was now well below me, and I did not feel comfortable going any deeper. I managed to attract the attention of our dive guide, who promptly swam down to retrieve my buddy.
The DM brought him back up to the group, checked his gas and computer, and he paired me up with a team of two girls for the remainder of the dive. The DM and my buddy returned to the anchor line, where they spent the rest of the dive hanging shallow, waiting for my buddy's computer to clear. Meanwhile, I swam around a shallower stretch of the reef with the other team. I was extremely grateful that the DM was there and able to step in to help out.
I felt really awful about not being able to help my buddy and not making an effort to communicate with him before it was too late. It was obvious from the start that this wasn't the right dive for the Go Pro, but I didn't feel right about confronting him on it. In retrospect, a lot of that discussion probably should have happened before we jumped off the boat.
My risk tolerance has changed since then, as I've learned that there's still quite a lot I don't know. Today I wouldn't dream of doing a dive like that with an unknown buddy, and I don't think I'd be willing to do it with a single AL80 either. Even for something shallower and more benign, I'm a strong believer in over-communication. Talk about your diving experience ahead of time, and make sure you both understand the plan before you jump in the water. Review how you'll maintain communication during the dive, especially when there are distractions like cameras involved. And don't be afraid to speak up if something isn't going quite right.