If your situation is true, that she wants to be shared but is too shy, it's the same situation I found myself in with my wife.
She would not take the next step, so I took a gamble and set her up on a SPAM.
She showed some kind of anger, but quickly moved onto Reading all her messages.
She then spent months talking about meeting and fucking him, but again, she could not bring herself to actually plan anything.
She wanted to, she fancied him, after all he was 20 years younger, and it had been 19 years since the last guy fucked her for the first time (me).
Again, I took a gamble and started talking to him. We arranged a scenario that could work, he rubber stamped it.
I simple presented it to my wife, and told her to decide if she wanted to or not.
OK, we wasn't too pleased when I told her, but two weeks later she met him for a drink on the understanding she could pull out at any point.
Two hours later he was fucking her in our marital bed....