Substitution Question

[Babyface]

Team A request/granted a 60 second timeout. After approx. 1 minute, all 10 players from both teams come out of their respective huddles and get into postition on the court. My partner is moments away from putting the ball into play when coach of team B sends a player B6 to the scorers table. I tell B6 that she will have to wait until the next opportunity to sub. Coach says that the first horn had never sounded and we should not deny her entry. I am certain that about a minute had expired, the timer just neglected to sound the horn. Should the substitution been allowed?