The mentor's role usually lasts for an extended period of time and possibly may not end until the person mentored reaches the level of the mentor. Whether informal or formal, both parties need to understand the parameters. The mentor's role is to teach and advise the new employee. The mentor does not interfere with the supervisor or manager's decisions. The new employee, while expected to seek the mentor's advice particularly on critical issues, is not bound to accept that advice.
Confidentiality is important.
Certain areas may be considered off-limits. The mentor needs to outline these areas at the beginning. Decide in advance how you will communicate. Both parties need to make their preferences known at the beginning and reach an acceptable compromise if the preferences are different. Discuss time limits.