Well, I dunno.. but for me, I have ranked my friends into a hierarchy.
A huge bulk of my friends are just mere acquaintance to me. For mere acquaintances, I do not really care whether they lie to me or not because I have no obligations to them.
But for worthy friends who treat me sincerely and honestly, I am willing to treat them likewise too and treat them as very close friends of mine.
Of course, I don't expect close friends to be totally truthful to me too. It all depends on the situation.
However, for girlfriend, I do expect her to be truthful to me. I can't imagine living with someone who's not in the future.
I guess for your case, it all depends on what degree of friends do you treat him/her. Keep worthy friends by your side but for those mere acquaintance, it's alright to know them, just don't get too close with them will do.