First of all your friend should have Maytag come out and repair the unit under warranty and request a copy of the service/repair order that the Maytag serviceman writes up.
Your friend will then know what the trouble was with the unit and have a better idea of whether the other repair company was out of line or if they were within reason on the cost of their repairs.
Now if your friend was just asking for an estimate for the repair of the unit, then the repairman shouldn't have performed any work without first receiving authorization. This authorization doesn't necessarily mean it had to be in writing, it just makes the issue of whether or not they requested for the unit to be repaired, an easier call for a judge to make.
Now if the guy replaced the fuse in the thermostat and it didn't correct the problem, why did they have him go into the attic to begin with? If they were only asking the repairman to replace the fuse on the thermostat, there was no reason to enter the attic.
Since we don't have any way of knowing what was said to the repairman or the company when calling them for a service call, we have no way of knowing whether or not your friend implied that they wanted the unit repaired by the company they called out and let into their home to look at the heater.
I can tell you that most people who schedule a repairman to come out on a weekend, do so because they have no heat and it is an emergency type of situation because they need their heating system to function. If the repairman comes out and fixes the system, they expect to be paid for doing so. They of course must produce an itemized bill listing all labor and materials/parts they had to replace. Since your friends called this company out because they didn't want to wait a couple of days for Maytag to get there and make any necessary repairs, I would think the broken heater situation was urgent.
I don't know what the details of the warranty may include, but if your friend didn't want to replace a fuse because it would void their warranty, then I would think that anyone servicing the unit, outside of an authorized Maytag repair person, would void the warranty. If your friends were calling out this company because they are a well known company and qualified to repair the unit, then they should have been able to recover their cost to repair the unit from Maytag. Regardless of what the warranty states, it can be different depending on the laws of the state where the unit is sold. Since the unit was under warranty and it stopped working at a time when not having heat creates an emergency situation, Maytag should have reimbursed them for the cost of the necessary repairs.
You didn't specifically say that this repairman did actually repair the unit, but if he came up with this 4 - 500.00 bill, I will assume he did. The amount quoted over the phone was no doubt a service call fee and any work performed over and above this amount would have to be paid by the party receiving the benefit of the work and/or repairs done.
If the repair guy was feeding them a BS story and replacing parts that didn't need replacing or not replacing parts he claimed he did, then that is a different story. But it sounds like your friend called this company out during "off hours" to repair their heater. Since the service call was roughly a hundred, the actual cost to repair the heater once they were at your friends home was 3 - 400.00, which is not unreasonable.
If Maytag came out and disputed that the parts that the other company replaced/repaired were bad, then your friend could request the money back from the repair company for anything Maytag claimed was done unnecessarily. If need be, they could have taken them to small claims court, along with the Maytag repairmen to verify the unnecessary repairs.
I wouldn't want to make a service call to someone's cold home on a weekend, climb around in their attic, replace parts/repair problems and then be told I wasn't being paid because the necessary repairs weren't approved. Why was they guy even called out if they didn't want the heater repaired by them?
Yet they are now claiming that the guy intentionally sabotaged their heater? So they have placed a stop payment on the check they did write, because this is what they believe may have happened? They didn't even wait to hear what Maytag had to say before not paying the company who came out at their request and in good faith to get their heat restored?
I'd say your friend is an idiot for jumping to conclusions without having anything to base their conclusions on. They should have paid the company who was willing to come out on the weekend to get their heat restored and recovered the cost from Maytag who would only repair their heater when it was convenient.
