Can you run 3s on the 7700

That guy was running a very small lipo. It's hit or miss running 3s, it could break down after a while, or last for years that way. It's probably not good for it, it's not recommended, but you can do it and probably get away with it for a while. Having a 7700 in a slash, regardless of the number of cells probably isn't a good idea to begin with, so he'll see heat and may or may not ruin the motor.