Can my client contribute a bonus to one employee's account because that employee has greater health issues?
No, employer contributions to employee Health Savings Accounts must follow Comparable Contribution guidelines as established by the IRS. Employers may contribute more for employees who are non-highly-compensated employees (non-HCEs) as long as contributions compare within employment categories. Non-HCEs are defined under Internal Revenue Code §414 (q). Contributions made through a Section 125/Cafeteria Plan do not need be "comparable" but must adhere to Non-Discrimination rules. (Notice 2004-2, Q-A #32, IRS 26 CFR Part 54: July 2006, Tax Relief and Health Care Act of 2006, Section 6, Notice 2004-50, Q:A-49 and Code of Federal Regulations, Title 26, §54.4980G: Employer Comparable Contributions.)