Although I don't know for certain her position, because rail tickets are not linked to a particular traveller, any form of ID or anything, it seems highly unlikely that they would be re-issued even if proof of purchase is to hand (with two sets, a fraudulent traveller could have twice the number of people travel for just half the normal cost). However, it is worth going back to the people/station she bought the tickets from and explain the situation - thus if/when the missing tickets turn up both sets could be presented in the hope of a refund of one (unused) set. This depends on the used set still being physically in her hands and not swallowed up by a machine somewhere.