Hey, I just found something useful here.
An employee has no reinstatement right if "each means of preserving the job or duties for the employee (such as leaving it unfilled or filling it with a temporary employee) would substantially undermine the employer's ability to operate the business safely and efficiently." 2 Cal. Code Reg. § 7291.9(c)(1)(B).
Being that you have only one employee, not filling the position during the employee's leave is an absurd requirement. That's how I would approach the problem if it were my employee.
Just realize that some enterprising employment rights lawyer may want to sue you just to try to get you to cave in. That could cost you money to defend -- so, having the law on your side, may still be a sub-optimal solution.
Re the amount of time that you must reserve the job, absent a legitimate defense (are you sitting down): it's 4 months!
Hope this helps.
And, if you need to contact me again, please put my user id on the title line of your question (“ToCustomerrdquo;), and the system will send me an alert. Thanks!