Should have reported him, it's an offence to catch and release greys isn't it?
https://www.rsne.org.uk/squirrels-and-law
If you catch one in a trap you have to humanely destroy it, just don't drown it - https://www.dailymail.co.uk/news/art...-squirrel.html - or take two tries and five pellets to kill it and leave it half dead for your neighbour to see while you go out shopping before finishing it off https://www.express.co.uk/news/uk/37...irrel-to-death.
We have loads of them up here, they are my main reason for wanting an airgun. If the wife is reading this I didn't mean it, I will only shoot targets, honest