Loss of an erection during sex can be as simple as -- thinking about that failing grade in Geometry -- and if it is an occassional event nothing to be concerned about. For women it would be loss of lubrication during sex -- again often brought on simply by distraction --. Take a break, change positions, go out and a grab a burger -- Nothing to be concerned about.

As mentioned before, if it happens on a regular basis it may be caused by medication side effects (over the counter, prescription, and street drugs), or another medical condition and your friend should see a doctor