Am i over reacting

So I just found out today my fiance was doing cocaine while I was working and he was watching our daughter…. I had a gut feeling something was going on so I went through his phone and his deleted messages and saw a guy named “big rico” and I read the messages and it was him asking to pick up so I was thinking just weed or something til I asked who that was and he said the guys real name and I know exactly who he is because all of my fiancés buddies use him to get coke… but anyways I’m like wtf seriously why are you doing that and he lied to me over and over saying it was for his friend but why wouldn’t his friend just text “big Rico” himself then after an hour he finally admitted of doing an 8 ball in the care of my 16 month old daughter…. I’m not the person to do drugs i know the affects I work with homeless youth who do that stuff and it goes downhill real fast and I would NEVER EVER put my daughter in that situation. My fiance also said it’s a one time thing I call lies What im getting at is would you leave or work around it??? I feel like I failed my daughter for protecting her safety because I know all those drugs are cut with fentanyl and the littlest bit could of harmed her but he just continued to say “well she’s fine isn’t she” I have never seen red in my life til that comment Please give me suggestions