For some people, the word is completely taboo - and for others, it's frequently used and means far less. I work with a doctor who frequently calls the photocopier this, and have members of my family who would be appalled and shocked if they heard him say it. In this context, because I know him, I just laugh it off when I hear him because the photocopier doesn't give a shit about the language he uses! Yet if he called me or another colleague it, he'd be out of a job. So it is a frequently used four letter word and the effect of it is surely determined by the circumstances it is used in, and the feelings people have about it's use.
We don't know the dynamics of the couple involved, and regardless of our opinions of the word, the lady called it is upset, and her friend, the OP doesn't know what to say to her, so wants our advice.
Mine, for what it's worth, is that the word is forgiveable if this guy uses the word a lot, about a lot of stuff. If he just uses it about her, then there's a lot of anger, rage, resentment and undermining of the lady concerned.
So, if it's a word he only reserves for her, then I'd tell her to talk to him about the issues leading up to the word, not the word itself.