Hello Dr. Mary:
You can try lomotil, as it is an effective anti-diarrheal agent in dogs. As I am sure you know, it is a symptomatic therapy, so while it will likely help, it may mask an underlying issue that may make it temporary.
As such, I would try dosing it, but also focus on Luca's gut as well by feeding an easily absorbable and digestible diet. Specifically, I would feed her boiled chicken and rice, served at a ration of 3 parts rice to one part chicken, served luke warm twice daily until you see the first normal stool...after this point, I would advise mixing the chicken and rice with Luca's dog food and wean her back to it gradually over a few days.
The dose of Lomotil in dogs is 0.15 mg/kg, administered up to 2 times daily by mouth as needed. I would not use it for more than 3 days.
If the diarrhea continues unabated or quickly relapses after you take Luca off Lomotil, then there may be some underlying mitigating factor such as an intestinal parasite, a common occurrence in dogs. In that case, i would get Luca to a vet with a stool sample in hand.