If they get a better offer whats the worst the current employer can do- sack them, so they just take the better offer, to be fair even if there is a clause in your contract the answer is not to let your current employer know until you get an offer from elsewhere
In my contract it's in the section with covers misconduct etc, I work for a fairly large corporate company so not surprised they have that in the contract
I work in the"real" world, In my contract I am in breach if I talk with prospective employers without notifying my current employer first, would have thought that was standard with most employment contracts