I consider my responsibility as a buddy to stay by their side, and I usually have more air than they do, but it's part of the job IMO. I go over that in my briefing with my buddy, I expect us to remain together and if one of us runs low what I expect.
In the beginning I was like you and would have considered this to be ok, but now I look at it as something that I will avoid. I leave the bottom with enough air to support both of us just in case, and my buddy does as well. Why take the risk, of course I'm preaching to the choir here.
I know you say you're experienced now and I did not see the bubbles, but you are aware that on many first stages it is normal to bubble a bit and if they don't you have a problem? Take a Sherwood Blizzard for example. As my buddy and I go over our gear I make a point to tell them that if they observe bubbles coming from my first stage don't worry, it's normal. I do point out to them the location the bubbles come from, it's not from the o ring tank seal area. But it is very close.
In any event, letting others know of shoddy performance is a good thing and I commend you on that. Many divers do trust me dives because they don't have the experience to know otherwise. I used to be one of those. Part of it is the classes we have, too short to really delve into the safety aspects like I think they should.
PS, did you go ahead and dive with them or did you and your buddy agree to a different set of rules?