The IRS defines a first time buyer as anyone who has not owned a home in the past 3 years. It doesn't matter that it's a mobile home. You currently own a home and would not qualify as a first timer. However, I've heard that there is currently a new bill under review that may extend the tax credit to all home buyers, not just first timers. Keep an eye out for that. Good luck!
Actually you cannot have owned a principal residence in the last 3 years. For example if you own a house that you have been renting out for the last 3 yrs and you have been living in a rental yourself you would qualify. This person does not qualify on the basis that she lives in the mobile not that she owns it.