I need help with dogs poop? updated stress colitis? [closed]

The dog I'm sitting has had healthy poops the first two days, but today he had an accident inside and it was runny, bordering diarrhea. It also had blood in it. Yesterday, he did strain a little bit with he last bowl movement, so I'm wondering if it could be a fissure? Everything I'm reading says upper digestive tract issues, but is not an emergency if dog is acting fine? Obviously I'm contacting the owner, but do I ask if they'd like us to set up a vet appointment? If they say yes, how does vet billing reimbursement work?

I'm also starting him on rice, and pumpkin and watching him closely the next few days. Is there anything else I can/should do? I'm still relatively new as a sitter and trying to remain as calm as possible, but feel really bad for the poor fellow. Thanks for any/all advice. ♥