Thank you for your patience and sorry for the delay. As you can imagine, there are a number of possible causes for Cass's diarrhea over the last little while. This could be related to the surgery, some medications she has been on since the surgery, or this could be a different issue altogether.
If this diarrhea has just started recently, then you can change Cass over to a home prepared diet for a few days if you are willing to do this for him. Cooked, boneless, skinless chicken breast and boiled white rice is ideal for this. The Purina he is on is pretty bland anyway, so if you don't have the time to prepare some freshly cooked bland food for Cass, then continue with ONE of the two Purina diets. Feeding both on and off could be enough to cause some diarrhea. Also ensure Cass has plenty of access to fresh water and continue to encourage him to drink.
If you can, double check with your sister in law when Cass is next due for his intestinal deworming treatment. If you can't contact your sister in law, you may be able to get this information from Cass's vet if you call them. If he is due, make sure you treat him with a reliable all wormer such as Drontal, Milbemax or Panacur. Because this boy has had heart surgery, don't give any other medication other than that specified by his vet. If this diarrhea doesn't settle after a day or two on chicken and rice, then you will need to collect a fresh fecal sample and take this along to his vet. Here the feces can be sent away for a fecal culture and fecal float to rule out an infectious cause of intestinal parasites. It would also be a good idea for Cass to have a full check up at that point, and if he is on any other meds, for your vet to re look at these to see if they could be causing the diarrhea.
I hope all of the above makes sense? If you have more questions or if I can help in any other way, please do not hesitate to ask! If you would like to accept my answer, please press RATE OUR CONVERSATION (I am not compensated in any other way). Bonuses are always welcome. Thanks! I hope to work with you again soon!
PS: If you have additional questions after you rate the question, you are welcome to request me for additional conversations if I am on-line or by beginning your question "Dr. E..." or "Pet-doc..." and others will leave the questions for me.