Whatever they choose, I hope it works out well for them. I just hope for her sake that it doesn't become a one-sided pass. For instance, while he's away seeing other people, she should be permitted to be seeing people on those occasions too. Will he be comfortable with other people — locally — knowing that their relationship is open? Or does he just want this to be a secret thing that he, and he alone, engages in when he has to travel for work?

Alternatively, if it's not practical for her to be dating while he's away (e.g., if they have a child that she needs to watch), she still deserves some opportunity for an outlet. Vacation-style, and away from him, like his. Not just trying to fit her extramarital jollies in around their mutual schedule and his convenience.

"I am going to sleep with other people, but you aren't allowed" would be pretty unfair, unless she was 500% okay with remaining monogamous herself.