What should a person do to mend fences and reconcile with his friends who abandoned him because he spoke with a non-observant person, which led to their boycott of him?

A Muslim must maintain friendships with good friends. If your friends have abandoned you because they suspect you are associating with disobedient individuals, you can frankly tell them that their suspicion is misplaced, that your conversation with the heedless person was fleeting, and that you were unaware of his true nature. Explain that you despise associating with such people and are keen to associate with the righteous. Clarify to them that you made a mistake and have returned to the right path. Ask them why this prevents them from restoring the friendship. Request that they acknowledge that they should have inquired with you and offered advice. If they are good people, they will accept, and your companionship will be restored. Otherwise, be patient; perhaps Allah will compensate you with other righteous friends.