I see you have quoted £16.90 for York > Sheffield which is the Anytime Day (SDR) fare, so I guess she is returning the same day...
Back in July a
Doncaster-Telford topic was posted and I found out it was cheapest to split at Cheadle Hulme/Goostrey, but this restricts you to the Northern stopper between Stockport & Crewe, so certainly loses flexibility. And it doesn't really work from York anyway.
A York-Telford ticket is routed Birmingham, and therefore set by XC and the Off Peak, at £58.80 is both overpriced and not valid until 0930.
The shortest route is via Manchester & Shrewsbury. A ticket to Shrewsbury is the same price as Telford (but valid at any time) and she could get a ticket between the two is only £4.90 CDR (£5.70 SDR). This is the quickest journey to Telford when leaving York shortly before the hour (usually xx:58).
There is no ticket that has "total flexibility", the closest you can come to total flexibility via both Sheffield and Manchester routes are perhaps a York-Chinley, and a Chinley-Telford (being valid both ways, but the latter is much further via Derby although the EC booking site will give options via Derby), but the latter is priced by XC so the SVR isn't valid until 0930 (but you can leave York earlier than that, of course). The SOR + SVR combo would cost £51.70. Not sure if this is valid for starting short from Sheffield at 0935 (which is the first train from Sheffield after 0930). It is a lot more expensive for two SORs.
If she doesn't mind being tied to going via Sheffield, your suggestion is good apart from you have quoted the SOS (Anytime Single) fare from Sheffield to Telford, the SVR (Off Peak Return) fare is £41.40 and valid after 0930, again priced by XC. So the first train she can get from Sheffield is 0935. So the 0820 from York, with a change at Sheffield, would be fine for your suggestion, but she won't get to Telford until around midday. Is this too late?
Before I go into this further can you tell me:
* what time is the earliest she can be at York station?
* what time is the latest she can arrive at Telford?
* can you confirm if she will be returning the same day or not?
I guess the return will be less time critical so whatever route works best on the outward she can return by?
I will then do a bit more research using NFM07.