I think its ok to be scared. In my house my hubs and I talked for months before we decided to actually add someone in our relationship.
To kind of help you here..Im also a wife who loved my hubs best friend. My now ex bf and hubs were best friends for many many years before I came along. So maybe I can help you from the other side of the coin that your not seeing..
My hubs is a wonderful man who I love more than life itself. I would trade him for all the chocolate in the world(and I love me some chocolate
) My hubs provides me with more love than I can explain. A wonderful father to our 2 sometimes crazy boys. (ok mostly crazy) When we embarked on this journey I picked his friend. Because not only would I have someone I loved but it would also be someone he related to. Could hang out with. And be comfortable around. It really isnt all about the sex. I wil lsay it was super hard for me to tell my hubby that I loved someone else too. But honesty is the best way to go no matter how hard it is. If anything after 12 years of marriage. I love him more than ever. We embarked on a journey with my now ex bf and I will say our relationship is better for it. I feel more open with hubby. The trust level.. just wow. There is a stronger bond between my hubs and I.
My hubs is my rock. My bf was the one who made me laugh instead of cry.
When my hubs couldnt be there for me while he worked etc. my bf was. I never felt alone to face anything.
Some nights when hubby was to tired from work and I needed...well sex honestly. I received that from bf. You know its always nice to have different perspectives on decisions etc...Well I had 2 that I trusted. So I could ask both.
My hubs had a friend he could trust to take care of me. My hubs had someone who could help him out when he was just to dang worn out. My hubs had time to do some of his hobbies while I visited with bf. Because honestly hubs hobby is not mine and to me its like understanding Chinese. which I dont lol.
We both benefited from this relationship more than words can supply. But communication is the key. Talk about it. Sit down and say look. Im really not comfortable with this can we talk about it for awhile. This is not something you just jump into. My hubs and I have been married 12 years. All mono. Its only recently we began poly. I have learned a lot just reading posts on here. And I think if your that uncomfortable with the situation than you shouldnt do that. She needs to understand your point of view and you need to listen to hers. Because it may not be as bad as you think. Dont think its shes plotting against you. But she should respect your feelings in the matter. I would have never done anything if hubs wasnt on board.