You get to know that Your best friend always lied to you (You got solid proof) !! and you also know it but ignores it in the larger interest !!
At what point you will prefer to take them heads on -I mean stand up and say Enough is Enough ???
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If a person is not truthful to you how can that person be your BEST FRIEND?? I do not think u should ignore it at any time.. Best friends are punching bags and they are supposed to be truthful to you at all costs. I would catch my best friend in the first instance and clear the air..