I (26M) found out the girl (25F) I?ve been dating for the last 6 months has a boyfriend (28M) of two years, should I tell him about it?

So back in April I met this girl, Sarah, at a friends birthday party. I started talking to her since we?d come by ourselves and both brought the same kind of tequila. We really hit it off, turns out we?d gone to the same high school but never met since I?m a year older. One thing led to another and we ended up going back to my place that night.
Afterwards we started going out pretty regularly, 2-3 times a week at first and then after a month or so we were seeing each other every other day. Everything seemed great and I loved every minute we spent together.
I should?ve realized it earlier but even though Sarah said she had her own condo, she always wanted us to come back to my house after our dates. A few weeks ago I was having some remodeling done and my home was a wreck, we had gone out for dinner and I convinced her to let me stay over at her place for a few days.
When we got there I noticed that it was definitely not decorated to fit her personality. She?s not a beer drinker but there was a beer cap coffee table in the living room and a minibar with a couple kegs inside. She told me it was for when she had friends over and I believed it.
I was supposed to stay there from Tuesday to Saturday but on Friday morning she woke me up at 6 and said I had to go since she was leaving for a few days to see her friend who?s fianc?e got hospitalized due to a car accident.
I didn?t see her for a week after that but she would still call me every morning and every night. Two days ago I get a text from her asking if she can come over. She opened the door and immediately started bawling when she looked at me.
It took an hour of embracing her on the couch for her to be able to say anything and then she let it spill. Turns out she has a boyfriend of two years and can?t see me anymore because he got in a car crash and shes going to have to take care of him until he heals. Apparently he traveled extensively for work which is the only reason neither him nor I knew each other existed.
She kept saying how guilty she felt, that leading a double life made it impossible for her to live with herself anymore but that I was the most wonderful person she?s even known. I just sat there dumbfounded at all of it while she apologized over and over for being such a terrible person, saying that she didn?t deserve me. I walked her out after we took a couple shots of bourbon and we haven?t talked since.
It?s taken a couple days to process this whole situation but now I?m not sure what the right thing to do is. I don?t think I can forgive her for that big of a lie, but I just feel horrible for the boyfriend. I?d absolutely want to know the facts if I was in his shoes so at least I could make a decision on what to do, even if it?s a horrible thing for him to hear. We we?re literally having sex in his bed when he got admitted to the hospital.
No matter what choice I make I?m going to feel like **** but should I break the news to him that his girlfriends been cheating on him for the last 6 months? I know it?ll probably destroy their relationship if I do, I just don?t know if I can carry that kind of burden on my conscious. As angry as I am at Sarah I still do care for her and I?m sure she?ll become even more distraught than she already is, but she knew the potential consequences for the choices she made.
What would you do in my position?
TLDR: Dated a girl for the last six months and she never wanted me to see her place since she shares it with a boyfriend of two years, not sure if I should let him know about the affair or keep it a secret.


Source.