garage wiring job

Got 2 12/2 cables running out an eave of a home and going to an detached garage. They are tied into the old wiring of the garage. Labeled “12/2 Anaconda M Durall T-type UF 600V”. There is probably 20’ of length running outside. I know UF is used outside but Can this be accepted?

“Two” is the first problem. You can only serve a second building with a single circuit or feeder.
Beyond that it gets down to whether the UF is sunlight (UV) resistant and whether it is subject to physical damage. The “damage” issue is always open to intrepretation.

It supplies one outlet, a garage door opener, a light with a switch. What should my client use instead?

If you need more than one circuit in a garage you can use a multiwire circuit (shared neutral) or you run a feeder and set a sub panel. Anything but the single/multiwire circuit requires a ground electrode.