Being the side chick??

Old friend from school is asking me to be his sidechick. He and his girlfriend are living together. I admit I added him back on snapchat first (last time we had talked he was single so I didn't know he had a girlfriend) but yeah they're literally living together and he is just using her for money. She buys everything for him. He keeps asking me to be his sidechick and said he has been attracted to me for years. We actually have dated in the past but things never worked out. I already told him no, but also I hate this day in age where cheating is so normalized. And yes I know cheating has always happened, but back in the old days they didn't have apps like Tinder and snapchat that makes cheating almost effortless. :/