You’re welcome, John.
There are some legitimate reasons for locating a water heater in the attic since it is usually warmer than a basement, crawlspace or garage (depending upon your location). The pull-down-stairs to the attic allows access…but is unlikely to hold the weight of anyone trying to remove a bad water heater. There should always be a catwalk built to the water heater for replacement and service with plenty of room for access to all sides of it for inspection and maintenance.
Being in the attic makes it easy for venting through the roof and, in most cases, it gives you much faster hot water distribution to your plumbing fixtures.
But as I said, the major drawback is water damage inside your home if the unit bursts or rusts out in the attic and the pan under the hot water heater can not handle all the water at one time. Many do not.
If it’s already there, you certainly would not recommend moving it but bigger drain pans installed under the hot water heater with high banks and extra overflow outlets to handle lots of water in case one rusts out or bursts is an excellent idea and should be recommended. In case of an absence of these extra measures, your client should be warned of their absence and the consequences of a significant discharge. Advise them to periodically test the drain to be sure that it is open. The water heater should always be well supported with beams under it and should have a plywood sub-floor under it as well and be well braced.
Without these additional safety precautions…as I stated before, your client should be prepared for major damage if the water heater were to fail in that location.