In strict registers, WHO acting as the object of a verb or a preposition is always cast in the objective case, whom. If you rewrite the clause without ‘pied-piping’—with the preposition for ‘stranded’ at the and—whom is still called for in formal writing:
Angela was curious about the unopened letter on the table and wondered whom it was meant for?
There is thus no error in the sentence as originally written.
Spontaneous colloquial speech, however, is less demanding; objective whom is rare in conversation (unless the speaker is working from a prepared text or is deeply experienced in the strict style), and almost never occurs at the head of a clause. Consequently, when you introduce the clause as direct quotation this properly reflects colloquial usage, not formal.