Allen:
Look at it this way, if she feels strongly that she wants to play...better that you are involved in the process than her doing it behind your back. By backing her on this, you two get to set up ground rules that neither is allowed to break without the others permission. If its behind your back...no rules...and no control.
Let her have her fun and satisfy her curiousity. And you can enjoy the leftovers
Allen has got it spot-on. My case was somewhat different. in that my wife was tempted by the attention of other men when she went out, but she turned down any offers of dates or casual sex out of loyalty to me. When we started using a realistic looking dildo, I was aroused by the thought of a new cock in my wife's pussy and we began to role-play and my wife's interest was sparked. Even so, it took a good 2 years of persuasion, encouragement and reassurance from me before Debbie gave in to temptation and, with my full knowledge and approval, went out with a young admirer who quickly talked her into his bed.
In hindsight, I think I felt that eventually Debbie would have been unfaithful, and my arousal at that prospect made me decide that, as Allen says, it was best I knew about it and had some input. We set ground rules (both of which she has since broken!) and agreed that if either of us was unhappy with the arrangement, we would stop.
You do need a strong marriage and trust for this, but it did make our marriage even stronger and improved our own flagging sex-life, so I would say give her free rein and enjoy the knowledge that your wife is being "naughty" (we don't regard it as unfaithful since I suggested it), satisfying her curiousity and it could quite possibly make her better in bed.
Peter C