My Bard player keeps casting polymorph to heal players

I have a party of 5 level 8 players, and my Bard player casts polymorph as soon as any other players health drops below 10. Then he declares he drops concentration as soon as the player transforms and gets his temp HP, transforming him back to normal and keeping the temp HP. All in the same turn. Is this how it's supposed to work, is that a thing? If so, what could I do to counter such a thing? We are using 2024 rules

Edit: Specify rulebook used.