This becomes particularly problematic when someone is vegetarian or doesn't drink in a group where the majority eat meat or drink. The cost of food varies and they feel it's not fair to pay more than what they ate. In the end, I think when you go out with a group, it's fine to split the bill evenly. If you don't pay for your good friend, who will. Alternatively, if money is an issue then just ask for separate bills. It's not worth losing friendships over money matters.