As others have suggested, pre-booking is essential. Try to travel off-peak. For example, I've just checked the fares currently being offered on the National Rail website at off-peak times on Tuesday, 5 December. (It's not usually necessary to book that far ahead but I wanted to ensure that I could find out what the cheapest fares are).
From around 0900 to 1300 (and probably at lots of other times too - I didn't check) there are plenty of departures, either travelling direct (with Hull Trains) or changing at Doncaster (for the GNER services). On all of those trains the ticket currently offered is the 'Hull Trains Pioneer Standard Advance 1 Single' or the 'GNER Standard Advance 2'. In both cases, the fare is just �15 (which seems reasonable to me).
Incidentally, if your friend repeats her mistake of buying a walk-up single fare, she'll find that the price will have increased to �72, on most services, by next month.
http://nationalrail.co.uk/index.html
Chris