Confronting, however gentle might not be met with good results. However, it is necessary if you wish to continue a relationship. Private confrontation is best at first... if they really can't see what you're saying, and others also care about them, then a larger intervention might help.

Start small. Choose something verifiable. (Do you have them on a message they left for you on the phone? i.e.) When they say something you doubt, like a date they will go with you etc... say to them, I'm writing that down now..and pull out a calendar and write it down. Then, if they lie about it later, you can remind them. Just a few suggestions, hopefully you will think of more. TC