S/he may have been ineligible for an inexpensive round trip fare, perhaps because of minimum stay requirements, so s/he may have gotten two round trips with longer stays and used the first part of each. As long as they're with different airlines, this is for all practical purposes impossible to detect. If s/he doesn't make a habit of it, and didn't get them through a travel agent who books this sort of trip a lot, there is likely no risk.