According to the WSR's a lag is bad if it crosses the long string (Rule 1.2b under general rules). Therefore, if the other lag is a good lag, it is the winner by default.
Link to rule
When looking this up I noticed another aspect of the rules which was interesting. It says that the players should lag again if one player's ball is struck AFTER the other player's ball touches the foot cushion. I'm sure I've seen this happen on several occasions but have never seen a referee call for a re-lag in this situation.