Not sure that this applies to your situation, but when the watch is removed from the wrist, it will pause a workout and also require a password. So if somehow the watch thinks it has detected a loss of contact with the wrist, that MIGHT explain those two symptoms, although my guess is that there could be other reasons as well.
Does she have good contact with the arm/wrist, is the band fairly snug and maintaining continuous contact with the arm/wrist? During certain types of workouts, a lot of arm motion or bumpiness (when running or bicycling) can causing such loss of contact.
I have noticed that unless I make sure the band is fairly tight during a bicycling workout, once in a while the heart rate stops updating, which I presume means that the heart rate detection has stopped due to loss of contact with the skin. However I have never had to log in again, I only needed to tighten the band and maybe reposition it slightly, then it all works properly. Now I just tighten it before the workout starts and don't experience that issue.
If the user has tattoos or some other unique physiology that interferes with the sensors under the watch, that could be a factor as well.
Is the issue with your wife's watch repeatable and predictable? If so, maybe taking it (and her) to an Authorized Apple Service Provider for diagnosis might be prudent. There could be something wrong with the watch itself.