Minutes before the bold disaster happened, Nida and Ranz were casually drinking beer. It was the last time Nida suggested drinking. Nida was actually an alcoholic. When she's stressed, depressed or hurting, alcohol is one of her escapes. When she's happy and celebrating, alcohol is also her partner. She's been alone for a long time, which is why she's used to relying on alcohol and craves it whenever there's a sudden spike in her emotion meter.
Now that she's celebrating being hired on the spot, she can't help but lift her mood much more with alcohol. The only effect of her alcohol consumption on her is that she becomes energetic and extremely talkative whenever she drinks. That is why she's not worried about drinking alone with Ranz. She still has the mind to act if something else happened to her.
She's been letting her guard down recently after living with Ranz and Randell and confirming that they're not a threat to her.