Forget talking to your brother. At this point, he is not thinking rationally. Once bitten by the girl bug, guys (from age 14 and up) will make every excuse to justify what they are doing, regardless of the damage they may cause.
The posters that said to contact Rabbi Wallerstien are your best bet. If your friend will not / cannot meet with him in person, have her listen to the shuir about being shomer negiah (avail on Torahanywhere.com)
Noone with a teaspoon of brains will hear that shiur and remain unchanged. Its one of the best shiurim I’ve ever heard on the topic, and applies to men and women alike