Simply being a part of a company from the start does not make you a founder, or even a partner. There are plenty of firms that pay in both salary and equity. Unless there was some form agreement, this person in my eyes is an employee.

What I would do in that position is sit down with the employee and explain in clear terms why they are not a founder or parter. If the owner is open to a partner, possibly offer up an agreement. However, I doubt there is any way to go over this without some tension. I would get it over with sooner rather than later, as the company grows it will only become more problematic.