substitution question

[just another ref]

I attended games tonight as a spectator. Team A has a double digit lead, but their star is on the bench with 4 fouls. Midway of the 4th quarter, team B cuts the lead to 6, so A1 gets up to check back in the game. About this time A scores a basket just as the lead official swallows a bug or something. He blows the whistle and stops the game for a few seconds to regain his breath and his composure. The other official beckons the sub in. Coach B protests that the sub should not be allowed during an official's timeout like this one, since this was not a "normal stoppage of play." (paraphrased) They discussed it briefly, then Coach A called a timeout of her own, making it kind of a moot point. After the game, since he knows I am a rules expert, (I don't know how to do the rolling eyes icon) Coach B asked me about the substitution. I told him I knew of no such restriction, but I would check my books, which I did and found none. I also told him I had a source that knew everything I knew and also most everthing that I don't know. (give yourselves a hand) To make a long story short, (waaaay too late) is a substitution in this case legal?