So you are saying that when she's on Deca she feels "normal" and off becomes depressed? What was she like when she wasn't on or between cycles - i.e. for her in general what does "normal" mean? Or is she generally depressed at times other than just coming off a deca cycle? Or is the depression associated with the peri-menopausal state & the deca "fixes" it, and then it returns post-cycle?
Here's one link I found on the subject:
http://www.bcrmh.com/disorders/menopause.htm
Is this something she's experienced with every deca cycle (not sure how long she's been cycling?)? If that's the case, then I might suggest that steroids tend to create a greater sense of confidence, some might even say "euphoria" - i.e. better, stronger, faster (just like the Six Million Dollar Man!) Then coming off, a depression, or at least a lack of that sense of confidence might result.
If the depression is related to the peri-menopausal state, she should be able to ask her ob-gyn for a more detailed explanation. If the doc doesn't know about cycling, just asking if depression is related to peri-menopause & why might help prove your theory about the progesterone derivative.