Yes, she is sad about loosing it because it used to give her joy and she likes dancing tango...she was also wondering if this can be 'normal' ie most tanguer@s experiencing it from time to time, maybe even preceeding a new development stage.
It's definitely very, very normal — as with any other discipline, the rate of improvement is not constant and fluctuates a lot. This is the same for enjoyment, for which a plateau often occurs when things have been the same for too long.
If it's about her own dance, well, there's always something that can be improved (and that something is easy to find). Is she happy where her dance currently is at? Does she feel a want/need to improve? Posture, walking, balance, fluidity, adornos, dissociation, musicality, et cetera.
If it's about enjoyment, has she tried traveling to an event, or even another locale's milongas? Maybe she's bored of dancing with the same old leaders. Maybe she just isn't getting enough of the dances she wants.
I hesitate to suggest taking up leading, but that's an option too if she really, really can't find any way to get over the plateau and still wants to stay in tango.
Taking a break for a few weeks is the easiest way to reignite the desire to go to milongas. After I came back from my two-week break over a month ago, I decided to go to less milongas (now 2-3 times a week, down from 4/5), and I'm honestly much happier and haven't hit a tango plateau since the last one two months ago. I would personally consider myself unhealthy if I couldn't stay away from tango for a period of time and if I didn't have other activities to replace tango.
If you could provide more details about her situation, we can definitely give more specific advice.