1st question would be WHY is the 27 year old still being claimed as a dependent?
If he has some sort of disability that hampers his independence, if that sort of classification of a disability is documented by medical professionals or the Social Security Administration, depending upon his age when disabled, he might be allowed to remain as a dependent on her employer plan or he might be eligible for Medicaid.
However, if he has little to NO income when he applies for coverage on the ACA exchange on healthcare.gov, he will be referred to Medicaid.
As far as using an HSA plan to pay for(or be reimbursed for) his medical expenses - that is a question either for your wife's employer OR the IRS in Publication 969.
It's Always Something . . . . Roseanna Roseannadanna