Say her yes as you have already allowed her to have a massage from another guy and that guy gave her orgasm so let her do the rest of the work. If you think that your sex life may get better then you can allow her to have sex with other man and at the same time tell her that you have also right to have sex another woman you like as you both are talking about open marriage where both the partners have a right to have sex with other people.
There are many couples are into open marriage especially in western culture and are happy so you can also try for it. Even if you say no to her then I think she will have sex with him behind your back as she already got the massage from him otherwise your knowledge and I am sure she also had sex with him but she is denying not to upset you.
Therefore, go for the open marriage but talk to her before stepping in the open marriage that you have a right to say no if you feel uncomfortable about it later on and she needs to obey you for the sake of the marriage life.