There are different categories of lies , of course, some told to protect others, some to protect ourselves, some to bolster our image, some to gain an advantage. I know someone who has lied for so long she really believes now that it is the truth.
I don't think lying is the worst of sins, unless it is done with malicious intent.
The question of whether it is our duty to tell our friends a hurtful truth is a very difficult one. My grandson was sharing a house with some friends, one of whom was cheating on his girlfriend. She begged my grandson to tell her the truth , so he did (he has Aspergers and never lies). She promptly told her boyfriend, forgave him and got him to blame my grandson, who had to leave the house.
As a child, if somebody asked him if he liked their new shoes, he would give his truthful opinion, which did not always go down well. My sister was the same and caused my mother great embarrassment on many occasions.
It is very true that liars need good memories and they often slip up. However, if their lie is not harming others it is probably better just to let it go.