This is an important point. I’m going to make the suggestion mentors, all of them, start charging for their time. The issue is that this is quite a common practise. Not just that, but people often use sites like PeoplePerHour, Freelancer, Upwork etc. by submitting assignments or technical tests and paying for the result, then submitting that result to the interviewers. It is fraud on a number of levels but one that does lasting damage to mentors as a whole.