The problem isn't how many strikes it takes to get to the penalty, it is the fact that somebody elses error, penalizes the entire group. I had a similar event where I was helping someone catch up by doing Battle on the Big Bridge. One of the members was in a long series of CS's dialog and couldn't get through in time. Because of that, I was penalized and had to sit out for 30mins. At the time, i was the tank, so the rest of the group just sat in DF for 30mins, then re-invited me and it instant flagged.

I loved the idea of people that withdrew getting a penalty when I heard about it, but only the player withdrawing should receive that penalty. On top of that, as far as I know, a preformed group of any amount will never be put into a group in progress, so they can't use the multiple players to abuse the system if it was that way.

TLR Don't penalize the entire party because one player makes a mistake.