If the strap is not in a good condition than the customer needs to be advised.
I don't mean if it is dirty, as you may insult them (although many times I've wanted to when they bring in a stinking piece of leather). I mean if the watch is about to fall off the wrist.

Look for:

the last one in particular as you are going to bend the plastic in a different way than the customer normally would and old plastic straps can fall apart!