First of all, congratulations and kudos for asking this Q. You’d be surprised at how many people would hammer the employee for the bathroom breaks without stopping to consider that the conduct may be the product of a medical issue or disability.
So, in a sense, in asking if there’s a “sensitive way” to broach the subject, you’ve answered your own Q. The answer, of course, is yes. Part of it is tone–the person who talks to the employee must show extreme sensitivity and tact. The other thing I’d suggest–and this comes up when employers suspect an employee of having a substance abuse problem–is to be constructive. Make it clear that you’re Q is in no way meant as an accusation and that the objective of asking it is to see if the employee needs help and not to impose discipline. If you do have an issue, we need to know about it so we can determine how to accommodate you. That’s what you’re looking for.
Bottom Line: Don’t back away. Your productivity concerns are 100% legitimate and you have every right to pursue this line of inquiry. Yes, there are legal implications in bringing up medical issues but you should be able to manage these risks by showing sensitivity and, to the extent the employee does disclose medical issues, follow to properly accommodate them.
Hope that helps. Glenn