It is possible that he discovered it was split and took it outside to fake breaking it whilst cleaning. I suppose we'll never know how the flood occurred.
Either way, the system should have been checked for ANY issues during the install and pump over should not have been possible.
If I were in the service guy's shoes, I would have been reluctant to continue servicing for a regular customer who had gone elsewhere for an install and, had I chosen to continue with servicing the new boiler, I would have gone over the whole system, charged for a thorough inspection and made damn sure to point out any issues to the landlord.