In my experience, which includes many years of frequent business travel, the savings on advance purchase non-refundable tickets outweigh the need to occasionally having to eat their cost. Non-refundable tickets purchased close to the travel date do not offer nearly the same savings: airlines know people's plans are usually pretty firm by then, and airline yield management people are not stupid. Besides, many non-refundable tickets have some residual value toward another flight on the same airline, so they're not a total loss even if plans change.

Your clients' likelihood of a change in plans may not be the same as mine was, but for what it's worth, I would not advocate buying refundable tickets in this situation.