As long as he is acceptable to her there shouldn't be a problem. If he is a wife hunter he will love it. It depends on his level of comfort to is he sneaking around with wives or open about it with the husbands?
It is supposed to be an open pass to experience other pleasure. If there is a possibility she will leave that is another situation. I have openly shared all of my life and when the relationships ended it was never due to another man but rather some internal issues. If her being with him could end things for you it would be better to not do it.
I would introduce him as a potential playmate foe her but maybe not state it directly in front of him. I would also do it in a way as if I thought she would enjoy a young handsome man depending on your age. I am still confused as to why it may cause a problem unless she feels as if you are pushing her on him for some reason.