Okay, that didn't generate any responses. So let me try it this way...
Are there any women in this forum who have experienced pain during sex which was not the result of a strictly physical cause (which had a strong psychological dimension)? If yes, would you be willing to share with us about if and how you worked through it and got to a place where sex was pleasurable rather than painful?
I'm sorry I didn't see this post like, a month ago.
With regard to your friend, I'd have a lot of questions and she'd have to be comfortable enough to be asked them, to honestly want to answer them.
First of all, for some of us, size matters but not in the way that many men seem to think or want it to. In other words, if you are "YUGE" then um...it's gonna hurt. Personally I prefer average-sized equipment on my male partners. I can do bigger when it's toys or hands because we are taking time, we can gauge how much is being inserted and so on... But when a guy is plowing away, galloping down the road to his own pleasure, and he's too large for comfort, it can become painful and unpleasant.
Also, some women experience serious discomfort when the cervix is "bumped" and size can matter, as can position, as can changes in her anatomical shape and size due to hormones and cycles, arousal and so on. I know that for a few days after my period, I can have some tenderness and pain might occur if the wrong area is probed at aggressively.
And then there is the psychological part, because the more a woman is relaxed and really enjoying sex, the body becomes somewhat more willing to accommodate. A bad headspace is not good for this. And a bad headspace could be anything from prior trauma or issues, to a not-so-comfortable connection, to self-critical body issues.
I would say that I had many experiences with my ex where pain was part of the picture, and not fun pain, and I dealt with it until he was done (thankfully that didn't take long) but I was not really enjoying sex. Part of that was size, part of that was psychological. I didn't feel emotionally safe enough to freely and shamelessly discuss sex with him, to try and make it better for me. I had walls up for my own protection, emotionally and mentally, with him. Sex was all about tending to his needs to "be a good wife" and even him asking me what I wanted felt like, "How do we make you want me, since my needs are not adequately met?" or "How do we make you enjoy sex so that my ego is satisfied?" It never felt like he genuinely cared about my feelings or my experience.
And if you feel horribly un-sexy, it can be difficult to relax enough to even try to be truly aroused or to have fun.
So I would be asking not only about the physical aspects, but also how much time and attention was given to her pleasure, how satisfied she was with it, how much experimentation and play was going on to discover what feels good to her, and definitely how she felt about the relationship overall, and how confident she feels as a desirable woman.
All things are possible factors.