He can't argue with the fact that he made a serious mistake, but he had 15 years of top performance reviews and the decision seems harsh.
So, he is appealing the decision. The general consensus seems to be that if the company believes he has a case, they will do whatever it takes to prevent a tribunal. If they make him an offer, how will he know if it's a good one?
The only way is to research what others in the same position have been paid out - check the press - tribunals often get reported.
He will also have to weigh up the risks - he could get to tribunal and be awarded nothing.
If he is seen to have committed gross misconduct or negligence, and was sacked for that, (and I'd have thought specially if it in any way compromised health and safety) and the company followed necessary procedures, then the company will most likely fight it.
If the dismissal is fair, he would get nothing at tribunal.
If it is unfair, they would give him a basic award (if you google statutory redundancy payment it is calculated the same way) and compensation for unfair dismissal. The compensation is based on his actual financial loss - it's not a penalty. So you need to think about how long he'll be out of work for, at a rough guess.
Any settlement will be formulated using likely award as the top number, but then discounting for how likely they think they are to win. You also need to factor in risk in the same way, plus getting the money now and legal costs.
That's how you work out if it's a decent offer. Try to take emotion out of it and don't be swayed by "ooh, X got £100k" stories. Those are rarely true and, where they are, normally relate to high earners with discrimination claims.