Just say you can't find it. You said you have an idea where it is, but you don't know exactly, and there's a bunch of stuff in the way. You don't have to explain, just say you don't know where it is. Not quite a lie, and can't be argued with.
I made the mistake of saying, "Yeah, I think I know where it is" when my uncle wanted me to give back a toy he had given to me when I was really little. He wasn't asking for the return of a borrowed item, he was asking me to part with something that had been mine for most of my life. It turned into a huge deal on FB private messages when I told him that this wasn't going to happen, and he changed his approach. What had been a gift suddenly changed from being "given" to "lent". His argument was that it had been his toy when he grew up, and he wanted his kids to enjoy it as he had. Well...it was also my toy when I was growing up, and it has sentimental value to me. I'd had no objections to the girls playing with it, but I'm afraid to even compromise and bring it up for his toddlers to play with at this point. I wouldn't put it past him to take it, saying, "I'm not stealing anything! It's mine!" The subject wasn't raised again after I didn't respond to his last message, though.
I don't mean to hijack the thread, so getting back on track. If she wanted to save the clothes for later, she should have held on to some of them in my opinion. If you don't know where the clothes are, and are too busy to look, there isn't much she can do.