Why are my reps getting a merge error when they try to access a Contact record?
Data Not Available You do not have Edit access to one of the two records you are trying to merge. Please contact your administrator to get additional access or merge the two records
Is it just a few of their Contacts or any Contact they own, also anything in the OWD for Accounts or Contact on your SFDC org?
I'd put together a a Report of all of their Conatcts and Accounts save it in a common folder, then go through the report logged in as them clicking Account and Contact names looking for a common link.
I have asked him to send me a few examples. He is the owner of the contacts and it is only some of them, not all. I don't think there is anything in the OWD.
So far this is the only contact that this is happening to.
Tier 3 ran the same update in your Sandbox. Can you check and see if she is able to view the contact there?