Hi, If your mother is in her 50's, this could be caused by different things.
1) may be a low level of B vitamins in which case shots would be administered and correct the problem (especially if you see the lying to be as a result of poor recall rather than trying to be deceitful). 2) Depression can cause concentration/memory problems as well as be linked to anxiety. You've mentioned about her missing turns. This could be because of fear of driving, or at least an anxiety attack. As a result, she is so consumed by her emotions that her mind is not paying attention to what is going on.
Could this also be as a result of her fear of getting lost/forgetting how to get back if she's going to an unfamiliar place?
3) In some cases, it is not unusual to start experiencing an early onset of dementia. How you handle the situation would depend largely on A) what you sense the behavior may be triggered by and B) her willingness to accept help.
If you feel that this is age related/memory problems, her GP can complete a blood panel test to rule out vitamin deficiency and other problems. If it is more of a psychiatric/neurological nature, the first step may be to see a psychiatrist. You have surely spoken to her about what is going on.
Try to take in her understanding of what she thinks is happening and why she is doing it. If this is a case where you perceive her as being untruthful rather than "ill", this too can be caused by different things such as changes in self esteem, social status, marital status/relationships, etc. Behavior is generally purposeful; it serves a purpose to the doer no matter how strange or maladaptive it may appear to the outside observers.