I think Pat's objections to Rob are
Mega dairy (so a bit of a kick in the organic teeth)
Protectiveness of Helen and Henry
Morality of the affair and the speed that things have changed since Jess left (she sees Rob as having corrupted Helen)
After all she only knows that he has cheated on his wife, no one knows about the rest of his treatment of Jess. So she could legitimately assume he would treat Helen well (once you've got past the capable of cheating part). And I suppose he could still.
But that conversation where he was basically saying 'if you interfere and things go wrong Helen will slide back into anorexia and it will be all your fault' was pretty threatening bearing in mind it's understandable that Pat is understandably concerned about Helen and Henry