Your son has a responsibility for the maintainenca nad prformance of the water supply and hot water system (section 11, Landlord & Tenant Act 1985). Unless you can 'prove' the tenants were responsible for this then your son will be liable for any loss to his property - or the tenants - caused by this. It is also woth noting that if the carpet was anything less han brand new, then a 50/50 split isn't 'going halves' because the carpet is worth a darn site less than a new one would be.
If the tenancy agreement said the tenants weren't to be away for more than (30?) days without notifying the landlord, and they were, then you may have a small chance of success, if it doesn't, you have no chance.
A tack I have used in the past is to say to the tenants, I will be recarpeting with this carpet (£2.99 carpetright). If you want anything different (better) then that is fine, but you pay the difference. If they say no, then you have got away at minimal cost. If they say yes, then you will have a better (longer lasting) carpet when they leave.