kamo ga negi o shotte kuru
A duck comes carrying a leek on its back
A gullible target/victim acts in a way that brings an ideal opportunity to the attacker, e.g. swindlers (due to their ignorance/naiveness)
In Japanese, カモ (“a duck”) often indicates a gullible target, like “a sitting duck” in English. This expression is quite likely the origin of the Pokémon カモネギ (Farfetch’d). Note that 背負う usually reads せおう, but its casual/colloquial pronunciation しょう is more commonly used for this expression.
Without knowing that she is a scammer, he asked for her advice on investment. This is exactly “a duck comes carrying a leek on its back”.