Haha, that's the worst idea you've ever had!
What?! This coming from a guy who thought [blah blah] was a good idea?!
Is there any way to translate the 'this coming from' into French? If not it's fine - I was going to settle with something like:
Quoi?! Mais c'était toi qui a pensé que [blah blah] était une bonne idée!