Can an agreement be one sided?
A pair agree to play benji acol (it's the first time they've played together).
One of them is a staunch believer in the old 8 playing tricks criteria. He opens such a hand with 2C, but doesn't have the strength required by BB7C1a. As far as he's concerned his bid is OK (it's not a misbid or a deviation). There is no suggestion that his partner has fielded.
They haven't discussed the system beyond "benji".
Is this an illegal agreement?