I know that my 17 year old daughter is cheating on her bf. What do I do?

She's been with her boyfriend for 4 years and they have a very solid relationship, but she's throwing it all away on a guy that I quite frankly think is a thug. She's at the age where she wont listen to me and will just ignore and disregard my advice, but I am deeply concerned. Her relationship with her bf has faced lots of obstacles (there's an age difference; he's 20 years her senior, and he travels a lot for his job) but they've come out stronger because of all of the sacrifices they have made. What do I do? I love my daughter so much and her behavior is abhorrent and should be nickled in the buck before she ruins her life forever.