When is the latest she is willing to arrive into London?
The cheapest option I can find is
Melton Mowbray - Nuneaton Route Any Permitted Off-Peak Day Return
£10.55 Y-P Restriction BX
Nuneaton - London Terminals Route London Midland Only Super Off-Peak Return
£13.20 Y-P Restriction LK
Use Oyster to get to/from Southall.
With this combination she is permitted to travel on the 1033 from Melton Mowbray and change onto the 1202 from Nuneaton, arriving London Euston at 1349. The only realistic return journey is the 1829 from London Euston, which connects into the 2052 from Nuneaton, getting back to Melton Mowbray at 2133.
A more flexible alternative is
Melton Mowbray - Nuneaton Route Any Permitted Anytime Day Return
Nuneaton - London Zones 1-6 Route London Midland Only Off-Peak Day Travelcard
£20.45 Y-P Restriction SK
This would allow her to travel on the 0733 from Melton Mowbray, connecting into the 0902 from Nuneaton, arriving London Euston at 1049. There is no evening restriction leaving London Euston with the last departure being the 1829 mentioned above.
If she wants to have an earlier arrival at / later departure from London Euston, she can purchase
Nuneaton - London Terminals Route Direct Off-Peak Return
instead, which would be valid on the 0846 Virgin service from Nuneaton (arr London Euston 0952). The last departure from London Euston is the 1907. She will need to use Oyster to get to/from Southall.
Going down the Midland Main Line, there is a large number of options. For example:
Melton Mowbray - Wellingborough Route Any Permitted Anytime Day Return
Wellingborough - Luton Airport Parkway Route Any Permitted Off-Peak Day Return
£12.50 Y-P Restriction B1
Luton - London Zones 1-6 Route Any Permitted Off-Peak Day Travelcard
£13.20 Y-P Restriction LS
This allows your girlfriend to take the 0733 from Melton Mowbray, changing onto the 0833 from Leicester and then the 0935 from Wellingborough (or changing at Kettering), arriving London St Pancras at 1029. A more convenient option is to get the train an hour later from Melton Mowbray and from Leicester so that a further change is not necessary.
She cannot return from London St Pancras between 1630 and 1901, with the last service before the evening peak at 1600, changing onto the 1723 at Wellingborough (or changing at Kettering), and then onto the 1818 from Leicester, arriving 1833. The only realistic service after the evening peak is the 1930, changing onto the 2118 from Leicester, arriving 2133.
The issue with splitting this way is that it severely restricts her options as only one service out of four per hour between Leicester and London St Pancras stops at Kettering and stations south thereof. The saving compared to a through ticket
Melton Mowbray - Southall Route +Via Leicester Super Off-Peak Return
£37.90 Y-P Restriction CI
is not much although it does allow her to arrive London St Pancras a little earlier than 1129 and she would not have to pay separately going to/from Southall.
If she needs to travel back in the evening peak then if you let me know, provided that she is happy to arrive London St Pancras after 1129, I can PM you a combination which costs £38.75, as opposed to having to pay for Anytime fares.