10 year old girl sleeping naked with 10 year old boy?

My daugher has a strange habit of being naked no mater what she is doing she is 10 so this is not aceptable to me about a week a go she had her friend jake over for the night I told her she had to keep a long shirt and underwhare on at all times I told her she may even have to whare pants if she got out of line well she was good all day so I thought it would be ok to let them sleep in the same room still telling her not to sleep naked and I would check on them sometime during the night well at about 1:00 at night I whent to check on them and she was compleatly sleeping in full view of Jake in the morning I told her what I had saw abd she told me she never did that trying to beleve her I told her ok but if I found out otherwise she was in troble about a day latter I got a call from jakes mom saying that Jake told he that Sara my daugher was sleeping naked and took off her night gown right after I had told her not to what do I do I told Sara I knew she lied and she admited to it but how do I stop this behavor for good