Mast looks fine to me. I've never heard of a mast suffering damage from tensioning, unless obviously kinked, something else gives before the mast does. So the mast is fine, do you know for sure the backstay tensioner was fully off?
If the owner uses too much backstay or other rig tension, he distorts the chainplates (topside dimples) or the hull (doors dont close or hatch slide binds, etc..) or breaks something.
Certified...in several regards...