Difficult feedback is more palatable when provided in an environment of mutual respect. You should work to ensure that your mentee knows that you’ll always be straight with them and that you’re looking out for their best interests.
As a result, they’ll filter any advice you give through an understanding that it’s offered for the betterment of their career. Your mentee will grow to understand that any perceived negativity is actually just a painful truth that’s important to process for growth and future success.