rat traps

A customer wanted a live trap to catch rats so I got her the wire live trap for rats from hav-a-hart and showed her how to set and bait it using peanut butter. She caught 1 rat and then one of her precious tree squirrels which was dead the next morning. This group of homes are all over 1 mil plus and give me big tips so I want to help if I can. But not being a rat trapper I am asking for advice. How can she catch the rats without catching the squirrels. Squirrels in this area don't always hibernate Thanks in advance for any help you can give.

Can't

Which species of squirrel are you having to contend with?
http://www.dfw.state.or.us/species/mammals/squirrels.asp
Which species of “rat”?
http://www.dfw.state.or.us/species/mammals/rats_mice.asp

Are you trying to catch them from an open area or a building?

A good Oregon Wildlife Species ID site.