FMLA is my life (or it feels that way some days). Per the DOL:
For planned medical treatment, the employee must consult with the employer and try to schedule the appointment at a time that minimizes the disruption to the employer. The employee should consult with the employer prior to scheduling the treatment in order to arrange a schedule that best suits the needs of both the employee and employer. Of course, any schedule of treatment is subject to the approval of the treating health care provider.
So, the answer is the ever popular, "it depends". It seems strange that this would be scheduled so far out. If it can be deferred more than a month, than it also seems like another 3 weeks or so is unlikely to make a huge difference, but I am not this person's doctor.
I also wonder, if this person is truly "running the store", if they fall under the key employee provision (top 10% based on compensation), in which case reinstatement isn't obligatory.