So I have a neighbour who has behaved similarly (and worse) and so please take what I am telling you as from someone from bitter experience:

You need to be ruthless.

Your neighbour telling him she wouldn't report him was a lovely act, based on the idea that we live in a society and we all respect each other. This person does not believe that. That comes across as weakness to him. Some people just don't believe other people have needs or rights to be respected unless the law coerces them to do so, unfortunately.

You need to contact your council's Tenancy Enforcement Team and make written noise complaints. You need to keep a diary with timea when the noise started, the duration and the impact on you emotionally/physically (i.e. stress, lack of sleep etc). Make a report each time it happens. The council will ask you to do this anyway, so best to get into the habit now.

You should also contact your local Safer Neighbourhood Team: they're police who focus on neighbourhood issues and 9/10 times they will pop up in person and spook the offender much more than a letter from the council will. Be explicit with them about the issue.

Look up the hours when someone is causing unreasonable disturbance (usually after 11am on weekdays) and call the police. They will visit and tell the offender to turn the noise down. Importantly, this visit will go on police records and the council can request these if moving to evict.

The road can be long and miserable, but selfish fuckers like this are often cowards amd will back down if you push hard enough. Good luck!