HSA option for an adult child of an employee who is employed at same company

Has anyone had success in enrolling an adult child in their own HSA when they are covered on their parent's plan? If so, please share how you accomplished. Or, if you have ideas of how to accomplish this, please provide feedback.

Background - an employee has a family medical plan that is hdhp and covers her son who is also an employee and 24 years of age. The parent is unable to use her HSA to cover medical expenses for child so the child wants to contribute to their own HSA and is eligible for the family HSA limit. The child will not be a dependent on the parent's taxes.