Hi, your dad its not wrong at all, you can take advantage of an existing finish but be careful with paint chips and deep scratches because new paint coats will tend to "copy" all defects from the surface, also you will have to be more conscious about lacquer/paint thickness.
First you have to make sure what is the kind of finish your guitar has...
Wheres how:
Test with a minimum amount of lacquer thinner, do it in the pickup cavities for example, if the finish dissolves then it is nitrocellulose lacquer if nothing happens or just the test area is a bit stained then you have a polyurethane finish or a polyester finish.
I highly recommend that you paint you guitar with polyurethane if you are painting a solid color, is easier to apply, is harder and dries faster than nitrocellulose lacquer.