Jumping to another company after signing the JO/contract

Hello,

I am writing on behalf of my partner. He recently signed a JO with a company and will be starting this 03/01 (50k). He then got a chance to apply to another company offering a higher salary (70k). Although he still needs to go thru the usual recruitment process, my question is:

Is it possible for him to not continue joining on 03/01 and instead jump to this new company in case he qualified? I am thinking if there are already existing legal bind on him even though he still did not start yet.

Ps i already asked him to review the contract.

Any thoughts would do… thanks!