I was speaking with a neighbour down the road who is a Sparky, I mentioned the OPs experience and he said he runs in to something along those lines himself but did bring up a good point.
If right at the point of service a customer queries the scope of work with differing expectations and then still goes ahead once the correct scope is clarified, he knows this has the potential for a negative review. He's aware (now) that people agreeing to the clarified scope does not mean they are necessarily happy, because the decision is made at the point of service, so there is some pressure on the customer to accept.. If they cancel, that can lead to costs being requested for cancelling, and also having to get new quotes and rebook someone else is going to delay things..
A good example he gave is when he's quoting for light fittings, he never used to include a bulb because everyone prefers brighter/dimmer or different colour temps and even style of bulb, so thought it better to reduce the quote and let them supply, but this wasn't clear and often was discovered at the point of service and unsurprisingly he had a few affect the reviews, one being very scathing.
The lightbulb issue made him change how he quoted, he thought removing the light bulb allowed for more competitive quotes, but in reality it's a false economy, if you set yourself up to be competitive like that, expect customers with high expectations and tight wallets occasionally, he now supplies bulbs, specifcally mentions this in the quote, and builds in a margin now to deal with customer service, something he accounts for and every quarter if there is enough in the pot, he treats his employees to something which also motivates them to focus on customer satisfation.
Of course you get chancers, and fighting those needs to be done on a case by case basis, however if they are asking for a partial refund and had at least queried it at the point of service, I don't agree with the strongarm tactics used, but pragmatically it might be an opportunity to use the 'good will' method and reply accordingly to turn it in to a positive for you..