While never steep, it is a steady uphill all the way from Cumberland to Frostburg. Since she doesn't ride much, it is probably too much for her. Conversely, the ride down to Cumberland is very easy. The tourist train will take you and your bike from Cumberland to Frostburg, then you can cycle back. Check their schedule at
http://www.wmsr.com/ . Since you want a longer ride than she, you could cycle up to Big Savage tunnel while she shops in Frostburg, then you ride together to Cumberland.