first i applaud your efforts to raise yr friend's daughter. she must have had confidence in her decision to ask you.
i feel that you all need to go in for family therapy too. she is testing you to 1. see how far she can go and 2. i believe her acting out behavior stems from anger prob because her mom is dead. she prob feels disconnected since she's now living with you all. how bout some family outings...only you all go..none of her friends yet. encourage her to have gf's over tho.
i'd have grounded her in a new york minute re the bra and pantie thing. you are now her mother figure and her behavior was way out of line. it's not necessary for you to tippy toe around her. she knows the skippy about right and wrong. right now in particular she needs a great deal of structure to help her feel safe and loved. BUT fair warnings just won't cut it, imho.
i do hope this will all work out for you all. guess i sound kind of harsh but my intentions are well meant. teens are rebellious anyway so you'll need to learn the skills to guide her. the therapy can help things fall in place. we all need to know there are consequences for our actions. she may be somewhat fragile but if she doesn't get turned around heartache may follow.
here's a site that may help you. page 2 is quite informative-
http://www.mayoclinic.com/health/par...SECTIONGROUP=2
hope you'll keep us posted. we do care.