*Telling* him what she was doing is  not illegal, so long as it is provably true--any person can tell any other person a true fact, so long as there is no written agreement (e.g. a confidentiality agreement) to the contrary. She can be angry at you, but you may legally do this; note, however, that if she feels that you may not be able to prove it's true, she could try suing you for defamation, which is making a untrue factual statement (a lie) about another person which damages her reputation. Note that if she is angry enough to sue, then even if you can prove the cheating and so would win the lawsuit, you'd still have to spend the time, effort, headache, and money to defend against the lawsuit--that is, if she gets so upset as to sue, even if you win the case, you lose to some degree, since you were forced to defend yourself in court.If the words you want to send (and you are right: do NOT send pictures, since that opens you up for liability) were sent to you--e.g. they are screen shots of emails or text messages to you--you can forward them to the boyfriend. Any one may forward any communication made to them to another, again so long as there was not some agreement to not do so. But if they were not sent to you and you instead took them off her private email or text or social media account, etc. without her consent, that is a crime.