Just 2 days ago, a "Friend" of mine assumed my boyfriend and I were lying to her. We told her we couldnt come out because it was his moms bday. So we stayed in. Well, my boyfriend accidentally left up the wrong away message...it read "Out with John". Long story short, she wound up leaving a really nasty voicemail which sparked a huge fight that potentially ended a friendship.
My advice is NEVER assume they are lying. I'd confront, but in a civil way. Ask flat out whats the deal.