RMDs are generally not required if still employed, even part time, at 70.5+ (never seen one that was required, but still used the word "generally"). With the exception being more than 5% owner, as mentioned already. Assuming the plan allows for rollovers, moving the 403b into the 401k would also defer RMDs on those assets for future years (ie, 2013 has an RMD on the 403b but not in 2014 if rolled in prior to 12/31). If they leave employment the RMD starts that year, so if they retire any time in 2014, then it doesn't help. But if they work a few more years, it's clearly a good move. You wouldn't want to do this with an IRA that had a basis, however, because you lose that basis.
EDIT: if the 403b has grandfathered accumulations (pre 87) which are already deferred for RMD purposes to age 75, then that's lost by moving it to the 401k.