Josef didn't try to persuade Mick to stop seeing Beth because he knew there was no point. Mick felt the same way about Beth as he did about Sarah. No amount of telling Mick he was setting himself up for heartache would change that. It needn't be that way, of course. Mick could turn Beth, if and when she were willing, if he weren't so pig headed.
But if Beth asked, Josef would turn her in a heartbeat. Mick would hate Josef for it, for a time, but he'd forgive him. And perhaps Mick and Beth could have the happiness he had been denied.