Is she a Silkie? (I can't tell from the photo for sure but she looks like she could be one.) Silkie's are generally great broodies!
If it were me, I would totally let her sit tight on the laid egg and anymore she might lay. Then you'll know in a week when your eggs arrive whether she is serious. If she is serious, she will sit on those eggs giving you an evil glare when you come around her. Set up water and food close by so she doesn't have to travel far to get them, but not so close the water can tip over and get the nest wet.
If she is still sitting and looking serious, when your eggs come, your risk is that she may stop midway if she has never brooded before and isn't that serious, or if she has been already broody for a long time and simply it is time to stop. (However very determined brooders will want to brood until they get hatched chicks.) Did the previous owner indicate why she would snap out of it?
The worse thing that could happen is that you give the eggs to her and she doesn't sit on them...or I guess she could also crush them or even eat them...so there is some distinct risk to the eggs since you don't know this hen. Therefore, if they are very valuable eggs, you'll have to be very watchful at first or I would put them in the incubator. If not so valuable, I'd give the hen a try and just be ready to quickly get them in the incubator if she is not being faithful. (I saved a batch by picking my Silkie up and setting her on a clutch that a not-so-faithful brooder had left...they were cold but the Silkie saved them. Not a great hatch, but I got 2 out of the 5 eggs.)
It's easy to just give them to her, place them by her chest and she'll likely just pull them under since you only have the one egg. Then take her egg away sometime when she is off the nest. Otherwise, gently lift her up and place the eggs under her after dark removing the first egg she laid.
Mark the purchased fertile eggs if they are not distinct from hers or others so you know which are your purchased ones. You may also want to let her sit on her first egg if there was a rooster where she lived before. Chances are it is fertile too...a freebie.
Good Luck
Lady of McCamley