All modern synthetics are better than the Mustang's V6 engine requires. Just tell your buddy to first make sure the meets the required specs listed in his owners manual, then buy the cheapest one.
I'd stick with 5w30 Motorcraft syn blend. $2.07/qt at Walmart and since he is bound to a maximum 5,000 mile interval for warranty compliance, there is no need for a more expensive full synthetic.