@So-chan She voiced her thoughts pretty clearly. She doesn't want to interfere with his social life, and do keep in mind that it's a way bigger deal in Japan than in most of the world. You're basically required to go drinking with coworkers and bosses to "improve relations".
The mixer itself is an outliner, and I 100% agree that he shouldn't go, and he shouldn't ask her and put her in a position where she has to say yes or no. Plus, mixers are for finding a partner, so if you're taken the person "paired" with you already wasted their evening, it's unfair.
But he did ask, and she was trying to be a good girlfriend. Again, you may not like it, but it's kind of understandable. I've seen how drunk people behave. If my girlfriend told me she wants to go get her ass drunk with friends I wouldn't like it. I'd spend the night worrying she might do something "cause she's drunk". But what am I to do, tell her "no, I forbid you to go"?