It's truly truly unfortunate that this happens. I have had this happen a number of times (even on occasion with close friends which I think is worse) and I have also often put in more money just to help the person who has to pay the bill out because it is not fair that one person bear the brunt of it all.
Sometimes people honestly don't realize how much things add up but sometimes people are just trying to get out of paying their share. Did you tell the person at the time that they owed more money? Although you would think we could all be adults about it, sometimes you just have to be direct about it.
I went to a birthday party not too long ago where a friend had a sit-down dinner with about 20 people. As the night went on people started leaving and would leave what they thought they owed for their portion. When the bill came and there were about 8 of us left, we all had to put in way more than we owed--there was no way we were going to let the b-day girl pay the difference.
Lately when I have been going out, I just open my own tab - it's just easier that way. I know this isn't always practical though, esp. when you are having dinner as well.
