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.