They don't turn off screen time on your sons device, the other friend just shares their screen to your son so they can use their phone for apps/communications where that is all being done on the other phone.
With Screen Time on your son's phone, you can prevent communication with this person so that will not happen. Of course, they could also turn to another friend that you would have to block as well. It is also possible to not allow FaceTime at all so that will not happen.
A willful disregard to your clear instructions is not something that is ever going to be controlled by technology alone, and you may even need to take the phone away until they can respect your wishes. If basic phone use is important, there are still "dumb" phones for around $20 with no apps/internet access that I bet your son would not want to be even seen with.