Apologies for the slight delay, I experienced some temporary connection issues earlier on. All seems to be resolved now so I can continue with my advice.
You are correct that he is not protected against unfair dismissal due to his service and the only issue is whether this would be a matter of gross misconduct, which would determine whether he is entitled to his notice period or not.
There is no specific legal definition of what GM amounts to but it has to be something serious enough which means the employee has breached their contract and are liable for instant dismissal. In this case, it would be potential GM if he had intentionally fixed his salary to a level he was not entitled to and had basically defrauded the company in the process. If this was a genuine error, not done with any intent, this should not really be treated as GM and a normal dismissal should take place instead.
If he had run the figures past his employer, they were checked and verified and the employer gave the g ahead then that would also be a mitigating factor. It would not work if he had also given them incorrect information on which they based their decision but if that did not happen then it would be a mitigating factor.
Regardless of the above it would also matter on what the employer’s investigation shows because the employer is only required to conduct a reasonable investigation and form a genuine belief so if as a result of their investigation they genuinely believe he had intentionally done this, they may still treat this as GM even in reality his actions were innocent. It is just that employment law does not have as strict a burden of proof as criminal law for example so the threshold to make a decision is lower. He may nevertheless appeal the dismissal and the GM label if necessary.
Please let me know if this has answered your original question or if you need me to clarify anything else for you in relation to this? Thanks