fornication has taken place, so is grounds for divorce. after finalised he is free to marry the girl, keeps attending meetings, gets reinstated. the girl studies and gets baptised when she is ready to and the congregation give her permission after going through the questions.
"Would it be considered a deceitful divorce because my brother is leaving his wife with the motive of marrying this girl?" - your brother was deceitful committing adultery with the poor girl.
"Although the wife would prefer not to lose her husband, she told me that once the divorce is final, she would not care if he remarried and wouldn't be mad. She would actually be as supportive as she could given the circumstances (but she would move on from him). The only person she does not want him to marry is the girl he committed adultery with. " - it is none of an ex's business who their ex marries next.
" He told me that he purposely committed adultery with the girl hoping that his wife would want to leave him, but she did not." - what a ridiculous thing to do!!! why not leave his wife, then do what he pleases with the girl? disfellowshipping is the conclusion to that too.
Sounds like he is more interested in continuing to live in the marital home, but without the wife, than being up front about anything.