nlp

I booked a ticket for my relative from RIC to LAS via DTW. The flight from RIC is to depart at 8pm on Dec 29 and she'd arrive in LAS at 12:13am. NW sent a mail saying the itinerary has changed and she will have to leave at 4pm and arrive LAS at 12:37am and will have to wait in DTW for 5 hours. I called them and was told that the 8pm flight is cancelled but they couldn't find anything later than 4pm and refused to put her on CO flights because of the type of the ticket. (The ticket costs only 415$). The only thing we can do is to change the DTW-LAS to an earlier flight so that she will stay in DTW for less than 2 hours. Has anyone had this problem before? Does it happen often?
Thanks..