Aya
This is Kawaguchi Hidetoshi. This is my humble answer to the question.
In particular, speaking ill of words is listed as one of the ten evils (murder, theft, obscenity, delusional language, bad talk, double language, greed, insult, evil opinion) in Buddhism. This has been addressed in the question below, so please refer to it.
Question “Who is someone who doesn't speak ill or complain”
http://hasunoha.jp/questions/113
Anyway, speaking ill of things is bad behavior, and it can eventually lead to bad results, so it's something you should refrain from. Therefore, you should be careful not to say it as much as possible, and if possible, I would like that friend to be careful.
However, even if you speak ill of others, the side being listened to doesn't really accumulate... Also, it's quite conceivable that it will accumulate as stress for Aya, so it's really inappropriate to keep listening for mental health reasons.
I wonder what is the cause of that friend's bad talk and despondency... I also think it can be thought of in various ways, such as dissatisfaction, stress, feelings of inferiority, trouble in relationships, etc. If you do something about the cause, I think bad talk and insanity will decrease... if you have an original personality problem, it may be quite difficult...
Anyway, when it comes to bad talk, it's a good idea not to talk to the other person, throw it away, or not accept it if possible. I have listed the story “The Buddha and the Bad Talking Man” in the question below, so I hope you can also refer to the fact that the Buddha dealt with bad talk.
Question “I can't believe people. I push myself in.”
http://hasunoha.jp/questions/369
Also, I think everyone's answers to the following questions with almost similar content (I haven't answered them...) will also be helpful.
Question “Tired of speaking ill of others”
http://hasunoha.jp/questions/74
Kawaguchi Hidetoshi Gassho