Update: Problem solved - 1st diagnosis was correct.
First, for whatever reason, this opener does not have electric eyes. I
personally inspected the situation last night. To verify that my dad had
done what I asked, I had him show me. He hit the lock button twice...
nothing. I tried it myself, being very deliberate, holding the button in,
then releasing it. Bingo. (Turns out it's all in the wrist).
So, he's back in business. Thank you all for your suggestions.