How do I stop mu 40 year old wife ftom sucking her thumb?

She might / you not. There are some interesting thumb guards out there designed to remind people when they put their thumb in their mouth & some prevent the mouth from closing around the thumb & interfering with suction.For anyone to benefit from such therapy they must decide they want it. If its a stress releaver, your efforts may make it worse. Hypnosis might be another option, again one needs to want change to do it.
Depends. When does she suck her thumb? When she's awake or asleep? If awake, it may represent a habit-and a psychologist or therapist could teach her techniques to extinguish the habit. Wife needs to want to quit for this to work. Interestingly, it is not uncommon to find adults who suck thumbs during sleep-and don't know they do it! consider dental appliances or thumb splints.