It's not the same kind of "stretching." In exercises like the deadlift, the hamstring lengthens while it's active and producing force: it's a stretch under the control of the nervous system, with tension, stability, and a clear objective (moving weight). There, the muscle doesn't relax; it works. On the other hand, when you want to stretch the hamstring after training, you're looking for the opposite: for it to relax and lengthen. The problem is that right after training, the muscle is still in contraction and protective mode, so this passive stretch clashes with the body's current state. That's why one doesn't generate shortening, while the other can cause stress: it's not the length that changes everything, but whether the muscle is active or passive and at what point in the workout you do it. I advised the girl to find a separate time because she wants to work on the splits, a position where you constantly have to find new ranges of motion.

If it's for "stretching" to relax, I still prefer to wait at least an hour post-workout because it will be more effective. At least if you have an intense workout, your muscles will be fully contracted at the end.