I[19M] recently found out my partner[18F] lied about meeting up with an online friend; what do I do?

My partner and I met online 3 years ago, however we only really began dating a year back, during this time we have met a couple times and things have been brilliant between us.

About 6 months back, I introduced her to my 'best friend' at the time who admittedly was someone who thought he was gods gift. Yes he had the looks, but he had promised me that he had no intentions of doing anything with my girlfriend. A couple weeks into their new friendship, he asked if we could all meet up, and he would show us around, sadly I wasn't able to meet up due to me being busy that week, however my girlfriend and he both weren't.

Fast forward to the next day and she had told me that they cancelled, I had no reason not to believe her and everything was great until the day that they had planned to meet up. I couldn't get in contact with her at all, and my so called best friend refused to talk to me, yet assured me over and over that he was studying.

Later that night when I was finally able to talk to my girlfriend she had told me that she had work that day, even though she did not work on that particular day. I didn't question it, and accepted that yes, she may have been working and he was studying which is why I couldn't contact them.

Fast forward a few months, I went to see her with her friends, had a few drinks and just talked. During that time we spoke about online friends and her best friend mentioned that she had gone to see my best friend. At the time I kept it to myself, but I feel sick to my stomach thinking about it now.

What do I do? She lied to my face and told me she was working, I no longer speak to him, but I feel betrayed. I trusted her.

Tl;Dr - Girlfriend of 6 months met up with my best friend in another city/state and lied to me by telling me she was working during the day. What do I do?