Here's a little on the technicality of the congestion is based on the load of callers online and what kind of usage as well. Most operators have their cell shared between 2G & 3G/data (MMS) services. For example if that particular cell by default, supports 30 time slots on a 2G basis, but if it is used for 3G it can only support 15 time slots, thus, if at that point of time there are 10 users sending MMS together, there are only 5 more 3G users allow to put a call through & 10 2G users to put their call through. So based on this logic, at the fireworks festival, I guess it is not only 10 - 50 users sending out MMS to their friends over there.
Your friend was really lucky to get their calls through, I had problems as well in such occasion when I was using Singtel services as well, one example is christmas & new year eve, SMS of all 3 operators are congested. I guess not many operators would want to have so much redundancy build into areas also as it is pretty expensive to have their cells capacity expanded for certain occasion.