That's simple.

Berelain uses her charms to get influence over men and manipulate them in the interests of Mayene. She did try with Rand himself, then with men of his entourage. She hit on Rhuarc this way, and he has plenty of experience dealing with this, as Aiel women deal with the chiefs the same way (and approve of women being forward). He puzzled her out right away,gave her a speech and took a liking to Berelain as a result, sees her as a daughter. he knew all too well why she was after Faile.

He's told what happened to Amys who shared it with the WO and they all took a liking to Berelain in turn - handling the hold/sept/clan chiefs, which often means marrying them, is after all part of what they train their apprentices in. The WO probably find that Berelain is a woman after their own mind, but is too obvious/clumsy at it, so they are advising her, it appears. That probably has a little something to do too with making sure she doesn't grab on Rand the hold they'd like Aviendha to take. As for Berelain herself, she is "networking". She's doing all she can to make herself indispensable so Rand keeps protecting Mayene, and developping excellent relationships with the Aiel is part of this. Part of her rivalry with Faile comes from this as well, and so is her coldness toward Egwene. She doesn't want Faile to grab through Perrin the sort of influence she hopes to gain herself.