I am sorry to hear that your girl is having these problems.
The short answer to your question is, yes, it is safe to use Diarsanyl continually.
If that is enough to keep her comfortable and control her diarrhea, it is certainly an option for you.
My concern, though, would be whether there is an underlying problem that is causing her to have continual diarrhea. I would suggest having X-rays and blood work run to help rule out major problems.
I hope this helps!
As always, if you have further questions or concerns, please don't hesitate to ask them using the "reply" function.