The trainer or client must provide 24 hours notice if they are unable to attend/conduct the session. All cancellations and rescheduling of sessions should be arranged between the client and trainer. If a session must be cancelled after the 24 hour window due to either inclement weather or personal emergency, be sure to communicate this with your client ASAP.
Please remember, this is a working relationship between two parties, so even if the cancellation occurs outside of the cancellation policy, it is important to try to work it out directly with your client. Many trainers will let the first one "slide" and be stricter with enforcing the policy for all sessions moving forward.