The woman of course has every right to refuse to remain in the company of the man after he has eaten onions, or can decline to go out with him in the first place if she knows or it is likely that he will eat onions before picking her up, or while they are dining at the restaurant. 

Once at the restaurant where onions are served however, she does not  have a moral right to physically prevent him from ordering or consuming the onions, or to insist that others prevent him from doing so, or that the restaurant be prohibited from serving onions.

Nor does he have the moral right to insist that she remain in his company while or after he eats the onions, or to physically prevent her from leaving, or to instruct others to prevent her from absenting herself from him.

The owner of the restaurant certainly has the moral right to determine his menu, and to serve onions to customers who like them.  The man certainly has a moral right to consume onions in a restaurant choosing to serve them. 

The woman on the other hand also has a moral right, to bypass the restaurant and to forego the company of the man, in favor of an establishment not serving onions, and the company of an alternate man who has not and agrees not to eat any while he is with her.

If it were possible to precisely show, define, and create a boundary for the onion smell, we might be able to apply this question to the principles of property.

In this instance, we are lacking as yet in sufficient technology.  The extent of the odor is determined in part by the acuteness of the woman's olfactory sense, and the extent of her tendency to become ill upon detecting such an odor.

In theory, the onion odor may extend for many yards.  At various points in space however it will no longer be detected, depending on who is doing the smelling, or what technology is being used to detect the odor.  The odor cannot be bound, and the odor's boundary cannot be easily or firmly established with any real accuracy.