What happens if someone reneges on a loner hand in euchre?
I have always interpreted the renege penalty as being generally 2 points because that's the maximum the opponents could have gained from the hand. In a loner hand, the maximum the makers can get is 4 points, so I would penalize a renege on with 4 points. Some sources:

On the Ohio Euchre page the renege rule says:


  RENEGE: - There are no exceptions to the ruling for a Renege: The hand is over, the offending team loses their bid and the non-offending team scores two points. In the case of a renege during a Loner attempt, the non-offending team scores 4 points.


On the wikipedia page for euchre, it says almost the same, though leaves open the possibility that other variants might not do this:


  In some variants reneging when a player or his opponent is going alone may result in a penalty of four points in order to equalize the potential values of a hand. The four point penalty for reneging should apply equally for the maker of trump and the opposing team.


A third source, Euchre Links says the same, though only specifically about the team that is attempting the loner:


  If the guilty team is defending against a lone-attempt during the infraction, 4 points are awarded to the team that was attempting the loner.


So, the sources seem to be in agreement that the maker would get 4 points when one defender reneges, and two of the three using language that indicates ANY renege in a loner hand (maker or defender) awards 4 points to the non-offending team.