This then is where good manners come into play.

The man, knowing that his friend is offended by this odor, and desiring her company for the evening, doesn't eat onions if he knows he is going to be with her.

In other words, in this case he temporarily acts for the sake of civility and good manners as if she as a "property right" that is determined by a boundary that doesn't really exist, and can't be determined.  He temporarily acts as though this boundary comes right up to and includes his own person, which it doesn't, and freely chooses to refrain from the eating of onions either before or during the date.

But suppose that the man doesn't know that he is going to be with the woman and has already devoured the onions?  Or perhaps he is at home and has prepared himself a dinner of onions, and is in the process of consuming them when the woman shows up on his doorstep and wishes to visit or to go out with him that evening?

In this case if the woman wishes to remain in the company of the man at this time, she acts as though he has a property boundary that comes right up to and includes herself, which of course is not so.  It is good manners in this case for her to bear up under the burden of the onion odor, and to ignore the odor if she possibly can.

This is all that mannerly behavior is.  When the property rights of the individuals involved are similar or cannot be reliably determined, good manners defer to the other person, in a fashion that simulates a perogitive or property right for the other person as if one actually existed, even though no such right or interest in fact exists.

Another example might be two individuals arriving simultaneously at a doorway that is too narrow for both of them to step through at the same time.  While neither has a preferential claim to occupy the doorway ahead of the other, it is considered good manners for one of them to step aside and to graciously allow the other person to step through first.