openingtree VS external-engine

If you know his account on any chess sites you can take a look at it on https://www.openingtree.com/ and find a line he plays that is bad for him, intentionally go for it and have some prep once you there.

I would recommend https://www.openingtree.com/ to start.

Might it be openingtree?

openingtree(https://www.openingtree.com/) will analyze hundreds of your games and let you see, move by move, where you tend to go wrong.

The 3rd thing is that the account is 9 days old. There is some really suspicious behavior here, including his best win being against a 2178 and his worst loss being against a 1214! (according to https://www.openingtree.com/).

Maybe use this website: https://www.openingtree.com/ and filter the games from a certain time frame, like you played that opening exclusively for a week, and then see what happened there, what were the most common positions, and that's the direction I'd take to learn from. Worrying too much about main lines doesn't have much practical value

https://www.openingtree.com/ contains both lichess and chesscom games. Here's Eric Rosen explaining how this works https://www.youtube.com/watch?v=AJ66-HqdpXE

Users can run a Stockfish instance locally, and then connect to it via UCI (Universal Chess Interface). Here's an example from Lichess: https://github.com/lichess-org/external-engine

lichess does now give you the ability to use an external engine over a websocket though.