How can a person help a bumble bee if it is dying?

If it's dying there's nothing you can do for the bee. However, if it's exhausted then the following is possible:-

Put the bee outside in the sunlight, i.e. a warm place (away from draughts) and feed it some honey. It will either recover and fly away when it feels strong enough, or die. I've rescued many bees over the years using this method.