If you break any companies ToS they have the right to terminate your service at any time, unannounced. If you break Facebooks ToS, do they email you with a headsup warning? No, they would just ban you instantly.
I partially agree with you if he would had suddenly started doing something wrong, but he was doing this for a lot of time and no one complained before. Cloudflare should give him the possibility to clarify or resolve the issue.
Obviously, in some cases immediate shut down is fine (eg. crazy resource usage, serving some illegal content) however doesn't seem to apply here.