Sub request during serve

[Scrapper1]

So I was R2 at a club event this past weekend. R1 beckons for serve and then a player for the receiving team enters the sub zone and the player he's replacing comes to the sideline to sub out. The rest of the team is basically looking at me to blow my whistle for the sub.

Meanwhile the server is ready to serve to a team that is not ready to play. As R2, do I allow the serve to happen for an easy ace? Or do I kill the play, disallow the sub, administer the delay warning and ask the R1 for a replay?

Basically do I allow the receiving team to lose an easy point because they screwed up the sub request?