As I understand it (someone more knowledge I'm sure will along). He needs to give notice to the new company as listed in the contract. If he gives less than this notice they could potentially pursue him for some costs to cover the inference in time. This maybe listed in the contract.
I doubt the new employer will do anything, they're not going to want to pay him wages for his notice period. No point having someone working for you for only a few weeks. It's annoying but it happens, I've known people just not turn up on their first day.