The problem with herbal supplements is that they are not tested to the same standards as prescription medication, therefore we have no absolute evidence base to give us information regarding side effects and interactions.
However I have been selling natural sleep aids with these ingredients for many years without patients developing problems.
Xanax is a drug where tolerance develops especially if she uses it regularly so she has two options with xanax. The first is to reduce the frequency that she takes it (perhaps alternate nights) or the second is to speak to her doctor about increasing the dose.
If her insomnia is a significant problem then she may actually be better switching to zopiclone or zolpiderm as more modern alternatives which are less inclined to be addictive.
I'm confused. "The first is to reduce the frequency that she takes it (perhaps alternate nights) or the second is to speak to her doctor about increasing the dose." I can see where increasing the dosage might help, but how would skipping a night help with the sleep?
Either way, as far as you know the Calms Forte should be okay with the Xanax, yes? (It's not been tested as vigorously as a med, but from your experience.)
The problem is that she has become used to the dose she is on. Over a period of time, taking less xanax means that she gets more benefit on the nights that she does use it.
If she feels that she could not cope with this, then she would need to discuss an increase in dose to get the same effect. But this can result in needing more increases as time goes on.
With the low dose that she is on at the moment, there should be no problem with taking the Calms but I am uncertain whether she would see any benefit.