Clearly she is going to be far over an hour late arriving at London if she stuck to WMT. Therefore, WMT are under an obligation to arrange ticket acceptance on Avanti (or if necessary, buy her a new ticket valid on Avanti) if she asks them to re-route her at the earliest opportunity.
If and when they refuse to do this (ideally documented in writing - easily done if requested by social media), she should buy a new ticket as necessary to travel on Avanti and claim back the cost from WMT. It's a bad idea to simply board the Avanti service and plead her case since the Train Manager is highly likely to charge her for a new ticket (as the rules allow, even if it's completely unreasonable in the circumstances), or if she encounters RPIs they could in theory take details for a potential prosecution.
Of course the alternative is to stick to WMT services (e.g. via Birmingham) and claim Delay Repay, which would result in an effectively free journey. There are plenty of alternative LNR services from Rugby or Milton Keynes to London so she's not going to be stranded - merely severely inconvenienced and delayed.