I see you are a contractor... me too.
Where I live, it's illegal. End of story. The liability is too high once I am aware to keep her on. And you have made it known publically that you are fully aware. I'd give a written warning, and prepare to replace her.
If it was legal (like it is for you), I'd have a different opinion.
The issue is if she handles information that could be compromised.
Perhaps she sees customers face to face, does banking, or handles their cash. Red flag.
Perhaps she drives a company vehicle occassionally. Big red flag. Even if she drives her own vehicle on company time, red flag.
Perhaps she prepares sensitive information, like tax returns, payroll, SS #'s, or handles secure data. Red flag.
Perhaps she gets face to face with customers who would be offended and will cost you business- yellow flag.
Maybe she collects important info that will cost you money if not done well, like subcontractor insurance certificates, licenses, etc. Red Flag.
Forget that she is hot. That is a red flag for you- sexual harrassment.
Yes, some stoners get detail freakish when high. Maybe that's an advantage.
Important question... how do you know she is doing her job well? Have you audited her?
The most damage I have ever seen to a small business was caused by the bookkeeper, and I've seen it dozens of times. Theft, carelessness, incompetence, or just plain stupid. I've seen bookkeepers walk away from a contractor with half a million in "redirected" funds.
Bottom line... It's a business decision. Make it.