How is the Friar at fault in the Canterbury Tales?
It's not clear what this question is asking for (At fault for what?). In the course of Chaucer's Canterbury Tales, the Friar and the Summoner have an on-going feud. Because of the lengthy prologue to the Wife of Bath's story, the Friar had interrupted the Wife. The Summoner defends the Wife and insults the Friar in the process, an insult which the Friar returns. Eventually, the host has to intervene and restore order.
After the Wife of Bath finishes her story, the Friar moves to tell a story about a wicked summoner (no doubt to retaliate for their earlier argument) whom the devil drags off to hell after an old widow curses this summoner.
The Friar's story about an evil summoner prompts the Summoner to tell a joke about 20,000 friars being concealed in Satan's arse. The Summoner goes on to answer the Friar's earlier story about an evil summoner with a story about an evil friar, who preaches with a view towards getting generous donations from those in his audience. When one of his generous donors becomes ill and does not get better, the upset donor wonders why the friar's prayers have not accomplished anything. After further preaching from the friar, the donor says he will give the friar something to share with his prayerful fellow monks. Unfortunately for the evil friar, the donor says he has hidden it in his arse. When the evil friar reaches for it, the donor farts in the friar's hand.
Thus, I suppose the pilgrim Friar is at fault for interrupting the Wife of Bath's story and for telling an insulting tale about summoners.