Home game probably thought I was a jerk

[checkmate36]

We had a small buy-in SNG NL game. 20 mins in a player goes bust (20 yr old) and the mom who was also playing says he can rebuy.

I said that was not established at the beginning and should not be allowed at this point. He can wait till the 2nd game and that can be a rebuy SNG if you guys agree.

The game has already started and we are 30 mins in and we should not change the rules. They thought I was an ahole.

Should the groundrules be established prior to dealing the first hand?