are you taking about a specific employee? for example if other employee are allowed to amoke on site during their break and this person isn't, unless there is a good reason - hygiene or safety then i can't see how ithout leaving themselves open to a discrimination case. cn you give some more detail, what do they do etc?
if its everyone then the employer has a right to make their site non smoking. assuming your not paid for your lunch break, what you do offiste during that time is your business