The pronouns are confusing me.
his return to see the psychiatrist who/whom she has already seen
Is that just a typo? Or are there three people, "him" "her" and the psychiatrist.
Another correction: Of greater concern would be...
Anyway, to answer your question, the correct word is "whom" -- he (or she?) has seen the psychiatrist, so the psychiatrist is the direct object. However, you'll see people use "who" there.