Of course she should try to pay for it. But I wonder if that would impact the cashier. It seems likely that the cashier tried to scan the comforter and then put it in a bag and it did not ring up. He/she may even have desensitized the security thingy or taken it off, thinking it scanned. I doubt this person walked out with an unbagged comforter.
It is surprising that the absence of such a large item on the bill wouldn’t have caused her to wonder that the amount she was paying was too low.
Is there any way the store would consider that she stole the item (or someone else did) if she tries to pay for it now? All she has is the item, no way to show that she tried to check out with it. Not a reason not to go back, but just wondering.