cummings66:I was diving with a new diver to cold water, he'd only done warm waters at this point. We did our dive plan and discussed various things. We hop in the water and down he goes, faster and faster. I tried to keep up, I'm on Nitrox with a mod of I think around 80 feet. We discussed the MOD, he was on air. He knew my max depth
DandyDon:I've had Air diving buddies go below my 1.6 MOD while I was on Nitrox - one of several reasons I don't like that combination. I advise them of my 1.4 MOD and my 1.6 MOD in advance, warn them I'm not going below 1.6 for anything, then don't - even when they do. Those are rare cases, but that's my stand.
The honor of buddy diving does not require forfeiting your safety to save one from stupidity. You do what you can, but that's' beyond the call of duty.
I don't know what EAN you were using...If you calculated your MOD at 1.4 then you would have been diving about EAN40 and at 1.6 about an EAN45...At 123 diving EAN40 you would have been pushing about 1.9ppO2. If using EAN45 you would have been around a 2.1ppO2. Oxygen toxicity risks are based on a time dose relationship. I don't know what your exposure time limits are on 1.9 or 2.1 ppO2. NOAA only goes to 1.6 with a 45 minute single dive exposure limit. Obviously you ended up ok, but if you look at the difference between a 1.6 exposure limit and a 1.4, the differences are substantial. I would assume going above 1.6ppO2 would further substantially decrease exposure times. You definitely overdosed yourself but obviously didn't hit the time limit for your body; or you were just lucky...Plus you were in cold water...BRRR

Your buddy would have been ok diving with air at 123 ft, but IMHO you put yourself at great risk going after him and could have ended up making yourself a victim.
I totally agree with Don...I wouldn't have gone after him as long as he was ok. If he wasn't...I couldn't answer that question. It would be dependent on the risk. But it is a VERY hard call when you have a friend in distress.
As far as waving you to the surface Shannon...If a new diver was diving with me because he or she wanted to be with a more experienced diver, I would NEVER wave them to the surface...When it was time for them to go up, eg, low on air, the dive would be over.