Its all depends on how the device is rooted, if the rooting is done by his friend without him knowing then it can be said that the phone is still safe.. sometimes this rooting can also be from a third party like where the friend got the device from.. their are companies that gives out phones on credit and for them to be sure that the loan will be paid back at the agreed time they root the device so that they can be able to lock, the device when the buyer fail to comply with his own part of the agreement. This kind of rooting I don't see it to be unsafe.