Your daughter would probably be best off travelling from Diss, buying an Off-Peak Return from Diss to Hayes & Harlington, and then an Anytime Return from there to Heathrow Rail. This would cost a total of £84.70 and would provide complete flexibility on the route and time of travel. The Elizabeth line provides a one-change option (easiest at Stratford, but also possible at Liverpool Street if the chosen Diss train doesn't stop at Stratford) so that's the obvious option for getting to Heathrow.
If she could commit to a specific train on the way back - which I realise is difficult due to the vagaries of flight delays - then it could be done quite a bit cheaper overall, as a combination of Advance tickets costs only around £22-25 each way booking this far ahead (again, generally splitting at Hayes & Harlington). I suppose one option would be to book Advances in each direction and then to change the time of the return tickets (subject to payment of the fare difference) if the flight is significantly delayed. In the worst case scenario, a series of split walk-up tickets for the return journey should cost no more than around £60, so she still be no worse off than buying the above combination in the first place.
However, this approach does rely on your daughter having internet access at the point where she realises she will miss the train though, so it's a question of whether the saving is worth the hassle. Noting she is flying from Terminal 5, the vast majority of BA and Iberia aircraft do have WiFi - but a handful aren't fitted with it and it's always possible for there to be a technical fault or a lack of coverage for part of the flight. Very slow WiFi (marketed as being for messaging, but also suitable for text-based web browsing) is free on both airlines if you create and sign into an account with them.
In terms of buying the tickets themselves, if your daughter does decide to go for the split Advance ticket option, in my view she would be best to do this via a retailer that doesn't charge admin fees for changes. Options for this include CrossCountry, TPE, Railsmartr and Ticketyboo. Of those, I believe Railsmartr is currently the only one to also suggest split tickets.
