I see these situations disturbingly frequently and they are truly heartbreaking.
There are a couple of things which may impact how much trouble she might be in and answering properly - giving all the possibilities and pitfalls - requires more information than is in your question (perhaps more than is available without making inquiries). For this reason, she really needs to speak with an immigration lawyer.
With regard to adoption, generally a child must be adopted before the age of 16 and be in the full legal and physical custody of the adoptive parents for two years. There is an exception for orphans, but absent more facts I couldn't determine if the exception would apply (not likely, but possible).
It needs to be determined if she has an outstanding order against her - you say "flagged for deportation" but I'm not 100% clear whether this means that there is a final order or something else - a lawyer needs to review the paperwork if it is in her possession, or obtain it if not. Otherwise, it isn't clear what the ultimate result of the USCIS arrest/detention was, and this really needs to be determined.
Can it be determined/documented whether she first entered the country legally with her parents, and then simply overstayed her visa (I know, unintentionally as she was very young then)? Do we have a copy of the passport/I-94 card on which she entered? If so, marriage to a U.S. citizen might work if it was something she was contemplating anyway with someone she was involved with. Of course this is not to be done just for immigration purposes, and is she does have an order there would need to be a joint motion to reopen the case with willing USCIS counsel.
Was anything ever filed for her pre-April 30, 2001 to obtain permanent residence (any family petition by any relative)? If so, and if that case was at least approvable when it was filed, she may be eligible for a penalty provision allowing her to get a green card here with some other type of current petition just by paying a fine - this would need to be explored.
Not sure if she would fall under the DREAM Act if it was ever passed. This is a proposed law to help kids in this situation - brought here and kept here without status by parents without doing anything wrong themselves. Gets introduced in congress all the time, hasn't passed yet and no telling exactly what form it would take if it does pass - she may be to old to benefit by the time it becomes law.