Maxperson
Morkus from Orkus
I don't know if the DM was just ignoring the rules or not. But in 5E there is no minimum, so you could roll everything well below a 10 and it would be a "valid" character.
Sure, technically it is valid. However, most DMs I've gamed with honor the spirit of the game, which is to have fun. They wouldn't let someone play truly crappy stats unless the person wanted to. That's not the same as re-rolling until you get great stats. It's just not letting the game be unfun for a player. Every once in a while, though, you get a bad DM who doesn't care about whether or not the players have fun. You should ditch a DM like that if he isn't willing to change.