As a Brit, but not a teacher, "leave well enough alone" means, to me, "don't interfere, in case you make things worse".
As such, it could be used in the case that you quote, but, beware of making things worse.
You may suggest that a person goes on a training course, that costs them money and doesn't bring in any wage/salary.
If he fails the course, he has not only had to fund the course, but has not had a wage coming in. He is therefore potentially far worse off (i.e. in greater debt) than he would have been if he had tried for a job rather than taking the course.
Hope this helps.
PS see also Answers.com:
leave well enough alone: Information from Answers.com