If your buddy was the R he did have the right to over rule you - he decided to count a goal when the officials (you & he) disagreed and he made a decision on timing by putting the extra time back on.
Good game management? Probably yes, since it's unlikely anything's gonna happen with 1.6 sec left in a 6th grade girl's game. Now if B would have stolen the inbounds pass & put in a layup to win...now that would have been something to talk about!
Anyway, don't let this ruin your relationship with your mentor. Stuff happens.