What if the person who received the free meal was very satisfied but noticed the line was moving really slow. Being an expert of soup kitchens maybe he could offer some suggestions that would allow the volunteers to feed even more people. Should he not try and help others to enjoy the same benifits
Sure, but there are a lot of ways of going about that. Some ways will work and be well received, and other ways will make people uncomfortable and undermine the "help" aspect of the kitchen for everyone.
If someone is a newcomer and doesn't like the way the line is moving and wants to help, they might be able to help more constructively if they take time to get to know the ins and outs of that particular setup, and over time get to know more about why some of the rules have been established. Maybe there are reasons for the way things have been set up, maybe there are conversations that have already taken place, things that have been tested and tried, and a consensus arrived at that works for the reality of what the soup kitchen deals with on a daily basis.
I think especially when people's emotions are so vulnerable, there just needs to be some extra layers of consideration added to the framework for everyone to get what they need safely. I don't think that's always apparent from the outside.