Hello, sorry to hear you have broken the 5th meta tarsal for the 3rd time. I think you should ask yourself (which you seem to have done) why it snaps. I assume it is a stress fracture and this will continue to happen again and again unless you get it examined properly why this is happening. The reasoning that it snaps because of tight Calf muscles seem to me a guess and I would like to see any research support for this claim. That it can have to do with the loading of your foot is most plausible though. Which means that at soon as you are on your feet again you need to get referred to a physiotherapist for biomechanical assessment of the ankle function (have you sprained this ankle in the past on several occasions?) . Maybe even better get yourself referred to a Podiatrist for this sort of assessment. With size 12 feet, are you tall, are you heavy built? If so this could compromise the loading. Poor foot biomechanics have a tendency to cause problems. E.G is your 2nd toe longer than your big toe? are you over pronatin excessively? are you putting all your weight on the outside of your feet? and so on.
Your friends might be right that a bone becomes stronger after a fracture but this most likely only true in the short term and even if this would be true in the long term (which would mean that more and more force would be needed to fracture the 5th metatarsal) the biomechanics is the clue and you might have to compromise in choice of footwear and insoles.
Hope this helps.