As glenp says get them to understand that sitting down is not an option … it's something even quite experienced riders will do as anxiety levels rise. They're adults so be honest with them, explain what will and will not make them crash, they'll either except it and continue to ride / progress or they won't.
Yes a skills course would make a massive difference to them … but I'm biased 😉
Have you considered that they may not trust your advice? Don't take that the wrong way but I do see lots of people who spent their time riding with good riders but they find it difficult to accept their advice … if that makes sense.
Stuart