From a selling point of view, the licensed sets are more popular than the City ones. Star Wars, DC, Marvel and Harry Potter are the most popular. He seems to have a lot of City sets. There are some Kingdoms ones at the top left which have been discontinued for a number of years. The older the set the more likely it is to be worth something. A good way to check the value of a set is to go onto http://www.brickset.com, type in the set number and see what they are selling for on eBay and Bricklink.
For example, 60052 Cargo Train (middle back) was released in 2014. It was discontinued in 2018. On Bricklink it goes for £148.45. The RRP for the set was £149.99. The average selling price on Amazon was £105 with a low of £90.
It looks like he has 7946 King’s Castle which goes for around £200 on Bricklink. It was released in 2010 with an RRP of £81.99.
With some effort, he could make some money from that lot.