When I worked in a restaurant during college, people were always trying to debate electrical and plumbing details with me.

I am not a mechanical person and know nothing about these types of situations. I doubt the waitress does either. The management staff was constantly giving the employees reasons for heating, cooling, plumbing and electrical problems and, when we would explain the situation to a disgruntled customer as it had been explained to us, we would have to endure the experts lecturing us as to what was or wasn't possible.

My suggestion is to leave out the accusation of lying. It isn't nice and, for all you know, she may have believed that's what the problem was. As JuliePie pointed out, she could have been told that.

Focus only on what the waitress is likely to be able to control.