Furnace kicking on in summer..

hello.. I have a 30 year old oil furnace has cold water going through and then out to a 40 gallon electric water heater.

My tenant is insisting on keeping the boiler off in the summer because he thinks it is just "wasting oil".

If it is off doesn't mean his electric bill will be that much higher because the electric heater will then have to heat and maintain the temp?

Aside from that are there any safety concerns about firing that thing up after it has been off for 3 months?

Any help would be much appreciated.