I don’t know of a verified ESL company that pays teachers directly in Bitcoin, but Preply may cover the flexible-hours part: tutors can set their own availability and hourly rate. For private students, Bitwage could be useful because it lets freelancers invoice clients and receive the payment in Bitcoin. She could also advertise private lessons within Bitcoin communities and accept Lightning directly.

Preply: https://preply.com/en/teach
Bitwage: https://bitwage.com/en-us/personal