If an employer has a qualified HDHP which an employee is eligible for but does not participate in because they receive benefits through their spouse's employer's plan, can they still take part in an HSA?

If the coverage an individual has under a spouse's plan is an HSA-qualified HDHP and the employee is otherwise eligible, the employee can contribute to an HSA. If the spouse's plan is not an HDHP or the employee or spouse is enrolled in any other disqualifying coverage (such as a health care FSA), the employee cannot contribute to an HSA.

Note: Whether or not an employer will elect to make a contribution to an employee's HSA if they are not covered by that same employer's health plan is entirely up to the employer.