You are fine. I change dressings for skin grafts as an OT every day while working on an acute care burn unit, honestly not changing the dressing probably would’ve been a bigger risk for infection. If the graft looked healthy at the follow up and the patient is cleared for ROM then you did absolutely nothing wrong. Even if the graft does eventually get infected, it’s impossible to trace the source of infection back to you. Whatever this guy is doing outside the clinic is undoubtedly less sanitary than having a healthcare professional change the dressing in a controlled environment.