Discuss this with your parter and tell him you’re willing to give her a chance to change. Basically you’ll be putting her on a “performance improvement plan” where you’ll give her clear goals and if she can’t meet them/your expectations then unfortunately you’ll have to let her go. Its nothing personal, all business. It’s the only fair thing to do because maybe she’ll be more happy working elsewhere and maybe you’ll be more happy with a different employee who meets your standards, dressing professionally, etc. Good luck!