This isn’t the news you want to hear, but you probably should tell him. Says dating and relationship coach Becca Hirsch, founder of the Not Your Average Bro dating website: “What if the tables were turned and it was your girlfriend who was on Tinder behind your back—wouldn’t you want to be told?”
And though telling him may make him hate you for a while, in the long run he’ll appreciate your honesty and loyalty
A note: If you do decide to tell him, don’t just blurt it out. “Be tactful about the situation you choose to bring it up in; and be sure to make it clear you’re telling him because you feel it’s your duty as a friend,” says Hirsch. “I’d also give him just the facts.” In other words: No editorializing, opining, or advising.
Also important: Support him in whatever he decides to do. “He may get angry and break up with her immediately, or he may decide to do nothing,” Hirsch says. It’s not really your business; so do the right thing, but don’t go all Judge Judy on his ass.