Like the Prioress and the Monk before him, the Friar is another representative of the religious establishment who fails to any of the expected virtues. Among other things,
He hadde madd full many a marriage/Of younge wommen at his owene cost. (ll.212-213)
[He arranged many young women's marriage at his own expense.]
He may sound like a generous man, but Chaucer implies that the Friar arranges these marriages because the "younge wommen" are his mistresses and, more to the point, pregnant.
Even worse than his illicit affairs, however, is that, as a person licensed th hear confessions:
He heard confessions pleasantly,/And his absolutions were easy;/The penance he required was light/When he knew there was going to be a good payment [to him]. (ll. 221-225)
The Friar's "selling" of forgiveness is one of the most serious sins the Friar could commit because he is subverting an important religious event--the confession of sin and God's forgiveness. Because God's forgiveness is supposed to be freely given, as long as the confession is sincere, injecting money into this ceremony negates its true value completely. Later, we learn that although the Friar knows tavern keepers very well, he has no acquaintance with the poor and the sick, another serious example of the Friar's failings as a representative of the Church.
In sum, then, the Friar is depicted as a religious man in name only, a person who does nothing but corrupt his own church for private gain and, in the process, destroys the foundations of faith among the people he is supposed to serve. Chaucer's portrait of the Friar is one of the harshest views of religious corruption in The Canterbury Tales.
In 'The General Prologue To To The Canterbury Tales' by Gepffrey Chaucer, the author tries to show us the human side of his characters. Most people, even today, are a mixture of 'good' and 'bad.' So we have some characters who have high aspirations (perhaps in terms of religion, serving God and man, or looking after others) but who, like us today, fail in that from time to time, because of what Chaucer might think of as 'original sin' but which we modern citizens might think of in terms of 'human frailty.'
So The Friar's main negative characteristic is his weakness. As a 'nomadic' priest with no residential obligations to the monastery, the friars was probably looked down upon during Chaucer’s time. The Friar seems picky over the people he chooses to help - pretty girls or wealthy gentlemen might be seen as a lucrative market for his 'business', so he might give the sacraments selectively, conducting weddings and forgiving the 'wrong' people'. In other ways, he may have become corrupt in the sense of commercialising his service to the Lord God and taking money for it.