Why are people so cruel?

My wife Enid was slowly but surely getting back her confidence to go outside via using a walking frame to get herself build up to go farther .But one day some arseholes  came past in a car and throw a doughnut at her and told her to go in you ugly fat bitch  .This event destroyed all the hard work we had done and put her at square one.She will not even consider going out now ,I have tried to encourage but the incident is deep inside her head,so words hurt and can cause a lot of damage.