Turning off the Gas would be classed as;
Damage with Intent to Endanger Life
Criminal Damage Act 1971 s1(2),
The criminal damage is not required to be visible, and so if the damage caused impacts on the functionality of the property, the offence would still have been committed.
Intending by the destruction or damage to endanger the life of another or being reckless as to whether the life of another would be thereby endangered;
shall be guilty of an offence.
Particularly as he's probably Gassafe registered (if he fits boilers) and would know the consequences.
You'd need evidence, fingerprints on the gas valve handle would do, but you still need to prove the gas had also been turned off.
The maximum sentence for criminal damage is,, Life.