If the engineer was there to perform an annual routine service on a boiler , one could put the case that anything else is outside his remit. Particularly if he is an employee of the service provider. Did sister engage the engineer ,or is it a contract she has with some organisation or other ?
Possibly, but were I that engineer and for my own peace of mind, I would at least do the quick simple check of wanting to know what section of the installation was the source of the leak. What if it was the boiler he/she was servicing?