Hide it? Mine didn't hide it. She was telling me about her new "friend" every chance she got. After a few weeks of hearing about him I realized he wasn't "just a friend".
I then confronted her about this new friend. I told her I was no longer comfortable with her spending time with him. That's when the real craziness started.
She decided we needed to open up our marriage. When I refused she said I was controlling!
I said someone's got to have some control here because you've obviously lost yours. That didn't go over well, as you can imagine

And the cycle continued for the next month until the dam burst on lake insanity.
You can easily imagine how this whole thing ended. She's now off with replacement and I'm left alone trying to clean up her mess. Fill in the blanks with all the typical BPD behavior you've read about. They're all so predictable