Hi MSFC, it seems there are a lot of layers to this so I'm going to go section by section.
'A friend of mine has a male partner who wants to engage in BDSM activity. She is reluctant ....'
First things first:
If she doesn't want to do it, well END OF STORY. If alternately this means 'she is interested but reluctant (because of the other issues) then maybe there's something to work with,
but 'Her view is that BDSM is abuse and an excuse for him to play out his inner frustrations on her.' Would say pretty loud and clear to me that SHE'S NOT INTO IT.
I'll give my two pence worth on why BDSM isn't abuse in a sec as I feel this is a bit of a secondary issue because IF
in their relationship BDSM WAS a way for him to play his frustrations out on her via what ever activity then it would be abuse.
The other point seems to be how are they dealing with these 'unresolved issues' that seem to be such a cause of concern to your friend?
In my experience taking up BDSM, or adding poly partners, doesn't resolve existing issues in a relationship.
I have NFI idea what 'his issues' are (I'm not asking you to tell me) but how do they deal with this stuff in their relationship generally?
On the BDSM is not abuse topic, very simply I would say: people have all manner of kinks, the line between 'vanilla' and kink is not that clearly defined, as long as FULL CONSENT is present from all involved then it can be path to intimacy, fun, sexual enjoyment.
'So, can anyone provide me with either a really good and basic source of information':
If she is up for buying books then I recommend Dossie Easton, the language is a bit California speak at times but accordingly her approach is about as fluffy and unscary as it gets. I notice there is a publication on her site 'When some one you love is kinky'.
http://www.dossieeaston.com/books.htmlxx