Or if the card's long, alphanumeric ID is hard to read/remember/convey, could the sellers simply not request the expiry date, which is in big letters (And gives the purchaser an opportunity to record it in their own diary/app, to prompt the parent's renewal).Is your mother not able to give you her railcard number for example if you have to purchase a ticket on her behalf?
If the trip (for self or others) is beyond the relevant, current one's expiry (or even purchase), there could be a tick box to declare this.
If the purchaser falsifies any of these specific declarations, they can't then claim the future absence of a Railcard is a mere oversight.