In the lesson , I think it is, "Tu avais pu leur dire au revoir....".
It is just , "You were able to say goodbye to them..." rather than just to say goodbye as you suggest. Just a bit more precise.
Hope this helps!
I am not sure what your question is. Maybe it's this?
Tu avais pu lui dire au revoir. -- You had been able to say goodbye to him.
-- Chris (not a native speaker).
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