HSAs are individually owned trust accounts. In order for an individual to contribute to an HSA, she must carry HSA-qualified insurance coverage and not be covered by any other plan that would pay expenses under the deductible on that plan. Her coverage is all that matters. Medicare is individual coverage and the fact that her husband is on Medicare does not affect her own coverage, so this does not present a problem.
Further, once the money is in the HSA, it can be used for any qualified medical expenses for her, her spouse, and any current dependents on her tax return for the rest of her life. So even though her husband is covered by Medicare and would not qualify to contribute to his own HSA, she can use her HSA to cover his out-of pocket expense when he is on Medicare.
Once she turns 65, the eligible expense list expands to include Medicare premiums as well, for any type of Medicare expect Medigap otherwise known as Medicare Supplement insurance. (Medigap is specifically excluded because it is in effect the opposite of HSA coverage it eliminates a deductible and shields the insured from any out-of-pocket expenses that are integral to HSA plans.)