What people have trouble being honest? If I'm around someone that has trouble telling the truth, I would evaluate why I tolerate their friendship. I have to be able to trust someone if I consider someone my friend.
Some people have trouble telling the truth because they were either too blunt when they were being honest and were punished, or they simply don't see the benefit of telling the truth. Either way, it's not my problem for long because I don't tolerate someone lying to me.