Pobre tonto. No se da cuenta cuando está siendo aprovechado.

English Translation

Poor sap. He can’t tell when he’s being taken advantage of.

I think you could also say “Pobre diablo”.

pobre diablo
m. coloq. Hombre bonachón y de poca valía.

https://dle.rae.es/diablo?m=form#92TlIS4

I had to look up bonachón:

adj. coloq. De genio dócil, crédulo y amable.

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You can’t make this sentence passive in Spanish because the person being taken advantage of isn’t the object of the active phrase. You have to say ‘se están aprovechando de él’.

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I’m happy you mention this, as your suggestion was actually the one I was expecting.

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