Make the property being delivered vacant a condition of the purchase contract. Maybe suggest to her that she offers cash for keys to facilitate. Ultimately she is bound by whatever lease she signed with them as would you be if you purchase the property and they're still living there.
As @Jon K. said, it's best to have the purchase agreement say the property will be vacant when you take possession. Obviously this puts the responsibility back on the current owner of the property and will make your life much, much easier in the long run. If the tenants are in the house when you take possession you may have to go through the eviction process which could add months (and added expenses) to your project. I'm not sure how landlord friendly your state is, but some states make it extremely difficult to get a tenant out regardless of the situation. Best of luck!
When you write up the contract include a contingency that the home be vacant upon sale. Let the seller figure out how to get rid of the tenant if it is something you are uncomfortable with doing.
@Chris Carreathers that is great if you can put in the contract to get the property vacant, but odds are good they want to unload with the tenant in place. If the tenant is gone, the value of the property goes up. If you are buying a distressed property, you normally take it as-is.
Give her two options. Give her one price for the property vacant and several thousand less with the tenant. Let her choose and setup the options so you win either way.