Its up to the employer to furlough, not the employee.
If the employee is in an at risk group, then the employer should make amendments or furlough (but doesn't have to).
If the employee is refusing to work, then the employer is well within his rights to not pay them, and place them onto non-paid holiday.
If the business is able to continue, and the employee cannot work from home, then the employee does not have a leg to stand on.