I cannot put conditions or definitions in the contracts that I give my employees that are not reasonable and being late for work cannot be reasonable described as gross misconduct.
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He wasn't late for work. He didn't turn up at all for his shift for whatever reason and they had to ring him. Failure to turn up for work when due and without notice can be put into contracts as gross misconduct. I have seen it plenty of times but I admit it is rarely enforced like in this case. The contract is important as the charge of gross minconduct or any other disiplinary charge will only stand up at tribunal if the disiplinary process outlined in the contract has been followed. If there isn't a disiplinary process or it hasn't been followed, the tribunal is likely to find on behalf of the employee. There is plenty of precedent in this area.
Taking this case on its merits, it sounds like you have two strong cases. First, you can argue that it wasn't gross misconduct which is certainly doesn't sound like considering your son has a strong employment record and secondly you can agrue your case about due process which in my opinion you are certain to win.
For a five star hotel or any employer to behave like this is disgraceful. Make sure your son stands up for himself and his rights.