If a peroneal tear occurs and could not be reattached, usually it would be sewn to the neighboring peroneal tendon, which in this case would be to the tendon that was reattached. Usually, this takes many months of rehabbing to regain strength. In the end, the strength would never be back to normal. Fortunately, there are other muscles in the foot that help the foot when walking. I would recommend orthotics for stability and a brace to help support the alternate foot while the tendons heel.
Thank you for that information and advise. Unfortunately they were not able to attach the 2nd to anything (it had shrunk back into the arch of the foot and the surgeon felt it would cause more damage to search for it then the benefits it would give). Since it has been seven months since the surgery could healing still be occuring? Also, what type of brace would give stability to the alternate foot?
Yes. Healing can still be occurring. It takes tendons a long time to regain strength and heal.
I would recommend wearing a hinged brace with an orthotic foot plate (eg the DonJoy Velocity ankle brace). He may benefit wearing this brace on the injured foot and an orthotic in the alternate foot.