Just get some OEM used 15" alloys in good shape off ebay, etc. The speedo and odometer won't be affected as long as you buy the right size tires (to keep the overall diameter the same or nearly the same as stock). Example...my stock 15's are nearly the same rolling diameter as my 18" wheels. All depends on the tire.

If you are worried about losing out on the relatively new tires that came with the car, just sell them on ebay as well to recoup a little money to help out with the new wheels and tires. Plus then you can buy the tires you want, and not have to just accept what came with the car.

Congrats on the new car, I bet she likes it.