
No law prohibits an employee from being reimbursed for more than his out-of-pocket costs in connection with business expenses, if that is what all parties understand and intend. As a general matter, the difference between actual cost and the reimbursement is taxable income to the employee and needs to be reported to the IRS. An employer which believes that it is reimbursing actual costs may have a claim against an employee who is padding expenses without the employer's knowledge, but the employer and employee are free to agree on such an arrangement if they want.
Other Answers
What he is doing is committing fraud if the company paying the bill is not aware of the markup. (I take it he is flying himself.) However, since yours is a private company, the federal Sarbanes Oxley act does not apply to you. I am not aware of any state law that offers protection for private whistleblowers. That is something you should check out with a Maryland employment lawyer. (I'm admitted to practice in Maryland, Ga. and DC, but I practice in Atlanta). You at least should make sure that you do nothing to assist him in his fraudulent dealings.
Sometimes, an anonymous note to your Boss's boss is enough. (i.e., Don't sign your name like you did with this note.)
Michael Caldwell
404-979-3150