Well my friend says that your friend should wait until the amount of water the plants are taking in goes up. For example if you are giving them 120 ml a day and all of a sudden the plants starts needing 250ml a day then its time to transplant. My friend also says that your friend should make sure he does the transplant no matter what at least 3-7 days before the change to flower so you don't have to transplant during flower and cause the plants stress.
Hope this helps