Is it really something that has come up much for MTB courses?
“Negligence” would be (I’m my opinion) failing to take actions to prevent unnecessary risks, or communication of potential risks as far as is reasonably practicable or foreseeable…
A failed (instructor supplied) harness or rope on a climbing course is probably quite a different scenario to most incidents that might occur on a MTB course, where participants will mostly be responsible for their own equipment, is it practicable for instructors to inspect participants helmets, brakes and tyres for adequate performance? Probably, but do they? I doubt it…
An MTB skills course should be booked on the basis of the riders own assessment of their ability, comfort zone and what they are looking to work on. Do many people who tend to prefer blues, book themselves on to advanced DH skills courses operated on black runs?
The decision to ride any trail feature will still be the participants own, under advisement from the instructor.
Prior discussions/correspondence of the course content, riders own ability, required equipment and trail gradings (ideally documented/recorded), plus a formal risk assessment maybe, should serve to highlight risks adequately enough to demonstrate the instructor was not negligent.