I’m sorry for your loss
However, I don’t think you can ask Let’s Encrypt to revoke their certificate directly.
Just like you can’t ask Globalsign to revoke someone else’s certificate when that website is performing fraud.
Let’s Encrypt is a Certificate Authority, not law enforcement agencies. You should bring this matter to law enforcement and/or online services (such as safe browsing).
Even if they are not allowed to use Let’s Encrypt, they can still request other CAs certificates.
Links to prior threads:
Disclaimer: I’m not a Let’s Encrypt employee. Just a volunteer.