You're doing the right thing by approving of her choices. If she's hiding this from her original boyfriend, she feels a great amount of shame and needs you to be supportive.
I would suggest that you get her in the right frame of mind for anal play. If her new Dom is sharing her, it's definitely in the realm of possibility that she will be DP'ed in the future.
I opened my wife's eyes about anal play by first teasing her taint with my thumb whilst going down on her, gradually entering her with my thumb and eventually using toys. She never realized that playing with her ass could bring so much pleasure. She now enjoys when I tongue her hole and often asks me to insert a plug during play. I've even made her orgasm by tonguing her backside.
