How openly do you two talk?
As in... do you know, for sure, the ins and outs of why she agreed to this? Is she interested in having sex with him? Is she happy, in her relationship generally? In herself? Is she allowed to sleep with other people, if she wanted to? What has she said to her boyfriend on this topic?
It doesn't sound like you think she is happy with herself. It doesn't sound like you think this is what she really wants. Those are both big problems, though the first doesn't necessarily indicate the second.
What's he like? Have you, prior to this, always thought he treated her well and made her happy? Do you like him as a person?
That there are people who, in this wonderful enlightened modern world, hear about open relationships and think that a) they should automatically get to have one and/or b) they're as easy as sleeping with someone else.
Neither of those things are true. At all. A functional open relationship takes a lot of work, and, in my opinion, works best either when neither party is massively invested, or, when they're in a very committed and healthy relationship with a lot of trust and honesty. I also think that they work better when both partners want to take advantage of the openness. They can work from just one side, but I think it's a riskier dynamic. Normally that works better when it's because one partner is no longer interested in sex. Or of course when the monogamous partner gets a sexual thrill from it - which it doesn't sound like your friend does.
I don't think you can do anything to stop this. But I do think you can encourage your friend to think about what makes her happy. What she wants. And definitely yes, be there if (I'm hesistant to say 'when', though I do suspect, from what you've said, that it'll be 'when') it all goes wrong.
It's all about what makes her happy. If she's made the decision that she'd rather be in the relationship with this arrangement, then that's ok. But it really depends on how she's come to that, and how he's making her feel about it.