That would not be a problem. The spring girl was blocked from doing normal challenges, no problem blocking a new girl from coaching, while still being the active event girl. Oh, and they now do no longer open their own Get Clean page. Previously, Mistlhoe and little Miss Sunshine could open that page by clicking below the TV. And winning with them does not open it, too. But I do not know, if it was previously possible to win and get trophys with them.
That sounds better. If it were a one shot, you would not get a message like "work on your best girl". That indicates a threshold. Other possibilitys were a cooldown, the said one shot, buggy implementation with all the clothingbonus and inherent girl bonus and whatnot.
But with that message, I say threshold. And there are several ways to do that, of course. All base stats higher, sum of base stats higher, higher in absolute numbers, higher in percent of number, combination of several aspects, counting in stamina somehow etc.
Should the threshold be around 600k difference in sum of base stats, my guess would be 14 days worth of training is the threshold - since it takes 14 days to coach an untrained girl.