While short leases aren't normally a great trade for you as a landlord, sometimes it can be difficult for a tenant to find what they need in a world of yearlong leases. With situations like these, it can sometimes pay to be a little flexible with the length of your leases. I can only imagine that they used to have a month to month lease when they moved into a new place, as it would have been impossible for him to figure out when he would be relocated again. Granted I knew them, but in a case like his I can't imagine why a landlord wouldn't have been okay with having a shorter lease. They were certainly a great family and always had a great time together, wherever they were. Sometimes you just need to follow the work, whatever that work may be.
Getting a call from a tenant like this may be stressful, but it isn't something to run away from if you can confirm that they need a shorter lease for something like work. Certainly having them list previous landlords will help to assure you that they will always come through with the rent, but we can also understand a concern or two on your end. This would come down to being your call, but we say you shouldn't count someone out just because they need a shorter lease.