Can I reschedule the biometrics appointment for my parents 2nd time?

Hi, I have applied for my parents for the replacement of the green card because their green cards were lost in the mail and they have not receive one but during the process they had to leave the country on I-551 stamp because of the sudden death of his sisters. He is heart patient and due to stress he is going through treatment and doctors are not allowing him to travel for couple of months. I have already reschedule the appointment and but after the USCIS sent the biometrics appoint again. will it be OK if I reschedule the appointment again? or they can loose their permanent resident status. please let us know we are so worried. Thank you