Probably all that she needs is a precise tune up including new cable housings and aligning the derailleur hanger.

There really isn't any difference between a 7-speed and an 8-speed derailleur so that'll work fine - provided the hanger and cable housing are right.

The shifter is another issue. 7-speed and 8-speed cassette spacing is just a skosh off so their respective shifters have a tiny difference in the amount of cable that they pull. Lots of guys say they've been successful in interchanging 7 & 8 speed parts but I've personally never been able to make them work to my satisfaction for very long at a time.