Coah asks for line up check after whistle to serve is blown

[officialtony]

HS Varsity match. Tean A is ready to serve when Team B's coach sees her girls are overlapped - she is desparately trying to corect this. R blows the whistle and signals for serve. She shouts to the U " line up check ! " as team A serves, the U walks out on the court waving his arms and states that Team B is requesting a line-up check ( ball is already over the net and in for an apparent ace ). R halts play and lets U check AND correct the line-up. R then signals for Team A to serve. Ruling please?