The Vedanta Sutras of Badarayana, Commentary by Sankara (SBE38), tr. by George Thibaut  at sacred-texts.com
43. But (they are to be kept outside) in either case, on account of Smriti and custom.
But whether lapses from the duties of one's order, committed by those who are bound to chastity, be mortal sins or minor sins, in either case such persons are to be excluded by honourable men (sishtas). For Smriti refers to them in terms of the highest reproach; cp. passages such as the one quoted under Sûtra 41; and the following one, 'He who touches a Brâhmana that has broken his vow and fallen from his order, or a hanged man or one gnawed by worms must undergo the Kândrâyana penance.' And good custom also condemns them; for good men do not sacrifice, study, or attend weddings with such persons.