Does your client realize it will look like this?
I suspect his/her reasoning is that the color will attract attention, which might be right. However, it will look awful. The color will scream out at the target audience, “Look at me! See how garish I look.” I doubt your client will want that to happen. Maybe you ought to talk to him.
Now if you’re a brilliant designer and can use this normally not-so-good look to design something amazingly creative, more power to you. Also, if your client has money to spend, there are 5-color printing processes that could achieve the fluorescent, chartreuse color in the background as a mask, of sorts, around the products or people. That might look pretty bad too, though.
My advice… You’re the designer; convince your client not to make a bad decision. Then again, I know nothing about your client or your client’s business — maybe garish is par for the course with him and his customers.