Two sides to this coin. It can be bad as the friend usually only knows one side of the story, as most of the time their friend will only divulge an edited story. The person is usually venting, super upset, emotional, and most likely not explaining it 100% objectively. That's why it's important to have a non bias 3rd party, like a therapist. A good friend can suffice instead, and can be a good source of support, but only if they know both parties and the circumstances very well and of course are very sound minded. I agree with OP in general, it's better to not drag people through the mud everytime you're going through it. On the flip side, people do need an outlet. So its different case by case.

To me a fair solution would be to yes, use your friend as support and sounding board, (but not to excess as I think that's just inconsiderate at some point) AND let them have a candid convo with your partner. That way they can act as a mediator between the two, and get a better more fair scope of the situation. A true good friend will want to be objective, as people inside of a relationship can let their emotions cloud their judgment, and they wouldn't want their friend to make a mistake they will regret, due to their own fault, something they were failing to learn or grow on. A good friend will consider the possibility their friend might need a straight talk, that could actually help them in their future, not just providing confirmation bias. Example: my ex refused to do anything without being asked, but also got angry when reminded to do something. They would get angry and their ability to communicate maturely or talk rationally went out the window. Then after they would forget that's how it started even though i fought to maintain mature calm dialogue. They genuinely seemed to be addicted to the fury and the drama. They would run to their friends afterwards crying about being unhappy, but because they couldn't even see what they were doing, there was no chance of their friend being able to understand, and in affect help the actual problem. i always felt if I could have told my side they would have been shocked at some of the details that my ex withheld or rewrote, and would have been able to help instead of just telling him what he wanted to hear.

(With very few exceptions) If your friend isn't willing to talk with your partner and just says "screw them" they don't have your best interests at heart; it's easier for them to just tell u to cut the ties.