Giving Admin access to your Facebook page is like handing over the keys to your car. There should be some level of trust, some level of mutual understanding and some knowledge that no matter how well your friend "drives" you're opening yourself up to more risk. If you are going to give someone access to your page make sure you take the following steps:
1. Select the right role:
Facebook allows you to select various levels of access from Admin (all powerful) to Analyst (simply has access to page insights but no power to post or comment as the brand).
2. Discuss expectations:
If you are going to give someone more access versus less be sure to define who is doing what so you aren't double posting or worse, assuming the other person will respond to fans / customer service issues. To a fan it is all one brand voice so you don't want to stumble over one another internally.
3. Limit access but not too much access.
You should have at least two people from your company as admins at all times in case one person quits or forgets his/her password. There's nothing wrong with giving agency partners access but once you've moved on from that relationship, be sure to remove them from the page. Same goes with an employee who quits or is let go.
You want to believe the best in people, and you should, but always remember to protect yourself and your brand identity at the same time. Too often people are too casual about admin access.