No, I would not use the Amox/Clav if it is that old if you have options... it would not be very effective at this point. If you can get Baytril, I would use that.
You can get a kitchen scale at WalMart for around $15.00 and it is an important tool to have for raising squirrels.
I think HRT is saying the Amox/Clav would be better for an UTI than Amoxicillin, but I believe Baytril (if that is what you meant by "bactril") would be even better if you have that option. The human equivalent of Baytril (animal med) is Cipro... in case your mom is a human prescriber and not vet?