Something came up the day of my drop-in for a client. Can I have someone else check on the dog for me if it is discussed BEFOREHAND with the owner?

I am dropping in for a nurse who works from early afternoon to midnight and her ideal time for me to let her dog out is between 7 & 8. I got called into work my full time job this evening and while I can still drop in, it is not at the time she is looking for and want to offer to have my fiance acomodate her for the time she wanted if it was allowed. Is it acceptable to offer to have my fiance check on her dog or is that outside of allowable actions as I am the one she booked on Rover? I