Thats why I recommended loctite on it,
Originally its a stretch bolt and needs to be replaced once removed - it wont hold much torque when re-used - buy some blue loctite thread locker and use it generously on the threads of the bolt.
btw - your mechanic telling you to throw the crank away if the bolt broke is a load of bollocks - At ZRS we remove broken bolts on the go - the tools used are called easy outs.. And have replaced about a dozen broken crank bolts for Honda D and B series engines