Is it normal after 6 to 8 years for stitches to come out from your surgery?

Permanent sutures can work their way out of the skin even years after surgery. No, it's not normal, but is usually not dangerous. Have your mother contact her plastic surgeon and let them know. Fixing the problem is usually quite easy.

It seems that from your description that a suture has gradually worked its way through the skin at the incision line many years after a tummy tuck or abdominoplasty. Permanent sutures remain there for the life of the patient whereas dissolvable sutures are gone in a few months (digested by the patient). Over time, permanent sutures may change position and work themselves through the incision where the coverage may be thin. There may also be a localized (limited) infection at the site because there is an open wound where the stitch is visible. By now, the incision is fully healed and the suture is no longer needed and may be removed by the surgeon without a problem. With cleaning and taking care of the wound and removal of the stitch, it should close in a few days to weeks without difficulty. 

It is my pleasure to answer your question. That could be a non absorvable suture coming out of the scar. I recommend her to go to the plastic surgeon who performed the surgery.