Academic job search: protocol for asking for letters of reference
First a bit of a warning. In many fields, the faculty from different colleges are likely to know and interact with one another at conferences and such. So it would be fairly normal, for a person writing a reference to correspond with another faculty member at your client's college. I assume she wants to avoid that situation, though it may be less important than she thinks. 

However, since your client is asking for letters from people known to her and who, in turn know her, it is also fine to include a note giving her current situation and wishes for confidentiality. Not prejudicing a future tenure decision would be a good enough reason, I think. 

However, the college(s) to which the recommendations are sent, might also, informally, seek additional information. This may be improper in some places, but it also likely happens. Thus, you can get no absolute guarantee that the current employer won't learn of what is happening. It seems a little harder for me to consider asking the new institution to keep it confidential. 

But, while I doubt that there would be a problem in any case, it would be good to have a backup explanation for "looking at another position while being perfectly content" in the current one, not to put words in her mouth.