The National Rail Conditions of Travel (Section 9.4) state:
"Where you are using a Ticket valid on a specific train service or train services (such as an “advance” Ticket) and you miss a service because a previous connecting train service was delayed, you will be able to travel on the next train service provided by the Train Company with whom you were booked without penalty"
I note this refers to a missed connection rather than a cancelled train.
The "Advance Tickets Terms and Conditions" (Section 8.4), at
https://www.nationalrail.co.uk/tick...tickets/advance-tickets-terms-and-conditions/) say: "If the train you purchased a ticket for is cancelled or is delayed and you still decide to travel, special arrangements will be made to accommodate you on another train (although a seat cannot be guaranteed)."
In practice this would most commonly mean the next train on the same route of the same company. I have experienced more flexibility than that being granted (by LNER) when there has been a more general disruption to the service, rather than just one cancellation, but it would appear to be discretionary.
I think it would be reasonable for your friend to complain, but my understanding is that Avanti are within their rights here.