2.3 volts on a cell is hard to come back from. I do have a old cheap charger that will cheat the cell and force the charge but this could be dangerous. Chances are that if you were using the balance lead to test servos, the one cell would be isolated from the rest to run the servo tester so you would have consistantly drained one cell on every use on the tester, leaving the rest almost fully charged. With the one cell down so much further then the rest you will have trouble charging it with a good charger. They have the built in failsafe for a reason, reason being it could cause overheating or fire trying to bounce it back.
I would say invest in a $5-10 cell checker just to be safe. Normally you dont want to go below 3.3 on any of the cells. Lipos aren't like other Lithium batts where you can drain them completely. They are only safe and effective from the 3.3-4.2 volts range. Lipos all the usable power is in the top 15% of the capacity, And it gets hairy anywhere outside of that. Not sure if the company will replace it, if you got it like that maybe. But if it was plugged into the servo tester the same way every time then you are the one who drained it from just the one cell. All cells fully charged and balanced should read as close to 4.20 volts, maybe an average of 4.18 but thats it.