The first thing I would do is thoroughly go over her past performance reviews to determine whether these issues have more to do with you now being her boss, or just her record in general. It's possible you're only seeing the problems now since you're her boss. And the asset you have now is access to these records. You really want to take a cold eye to evaluating her and take the fact that she's your friend out of the equation. This will help you decide what needs to be done to improve things.
Next I would give her some task or project to carry out with some clear expectations and deadlines. And then you can begin to evaluate her on how she handles those tasks. You're looking to see how competent and enthusiastic she is, and what level of support she expects from you, which may be unreasonable. Then you can take all this information and have a straightforward discussion with her about her overall performance and what needs to be done to change it. All of this should be done in a supportive context, making it clear that you're working for her success and it's not just a case of you being the boss now and wanting to evaluate her.
I would also acknowledge that your relationship has changed because of your new roles, and tell her that you need to establish some new parameters. Some questions to consider discussing are: What's within work and what's outside? What can be talked about and shared, and what can't? And what are the new expectations? You might explain that if she really wants to be successful there, she has to earn it in a way that does not smack of any favoritism.
Once you've had that discussion, then you're free to talk frankly later on if she continues to underperform. I would take notes on your communications throughout this process, and if it came to it, I'd lay out the facts and point out how the gaps in her work are going to affect her reviews and future at the company.
Mixing friendship and work is undoubtedly a tricky proposition. If you truly are her friend, you'll want her to do well, but keep in mind that as a boss, you make exceptions at your peril. Good luck.
Send Ron your career and job-related questions.
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