I have no idea, but I just got another controller used in the mail and both it and the original controller I had both work fine on 7.51 and I was able to sync the new controller to the PS4 on 7.51 no problems. I googled "7.51" ps4 controllers wont charge and find nothing. So I have no way of knowing what they're talking about. All I know is mine works and it's not a known issue like they're making it sound like. I'm not saying they're lying, I'm just saying it could be their PS4 or controllers.
Last of Us II 1.02 patch requires 7.51 so if there was a big issue millions of people who bought that game would have problems. It would be big news if 7.51 caused this issue but there's nothing I can find on it. So it's not a common issue it's something specific to them. Sounds to me like it's their PS4.
Since they're your friends why don't you tell them to either come to where you live and bring the controllers or you bring your Ps4 to them and test it then you will know for sure that's the only way or if they have a 2nd PS4 where they live to test it on it.
"I think they tried" is not a good answer you should ask them to try to charge them anywhere except the PS4.
