You can keep your miles from expiring by making a purchase through one of the vendor partners of the airline. I'm not sure about LH, but the OP's girlfriend can keep her miles from expiring for another 18 months by simply buying a 99 cent Itunes song through the United Mileage Plus Mall.

Also, if you book the ticket while the miles are still in your account, you can book the award ticket date for anytime up to 330 days after the miles would have expired. (in theory)