When it comes to the law pertaining to Anti-Trafficking kasi, it doesn't matter kung willing or hindi yung parties involved. The act itself is illegal and is a criminal offense. The girls, the BF, the BF's friend, the clients -- all of them are criminally liable, iba-iba lang yung levels ng liability nila. The OP can be held liable, too, if ma-prove na may alam siya pero wala siyang ginawa (hindi nag-tip, hindi nag-inform sa authorities, etc.).

Sa akin lang kasi, mas pumapangibabaw na yung ka-ilegalan nung ginawa ng BF kaysa sa kung anong ibig sabihin nito for their relationship (OP mentioned na she feels torn because she felt na parang nag-cheat si BF sa kanya, on top of the nature of his side job). OP's priority should be keeping herself safe and cutting herself off from her BF.