When will she be travelling? The way to get the best fares is to book well in advance, travelling at off-peak times.
If you enter your required dates and times for travel on the National Rail journey planner, it will show all available fares:
http://www.nationalrail.co.uk/
An Open Return (which has no limits on the times of travel) costs �249.00. A Saver Return (which may have restrictions on the times of travel) costs �99.30
If your daughter buys a 16-25 Railcard (�24), she'll get one third off of those fares.
http://www.16-25railcard.co.uk/
So a Saver ticket would cost �66.20 (making a total payment of �90.20, but the Railcard is valid for a full year so she'd make further savings on future rail travel).
Looking at some sample dates for advance purchase travel (out on 01/09 and back on 04/09), the National Rail website is offering �35.00 outbound and �27.00 back, making a total of �62. The 16-25 Railcard is also valid with all Standard Advance tickets, so that would come down to �41.30 (plus the �24 for the card).
Chris