They can locate the separate confirmed reservation, and ticket the luggage through to DEN. However, she will have to collect the bag at ORD and re-check it immediately after customs.

If she is going to take an earlier flight, I would imagine that she can ask the United agent immediately after customs to transfer the bag to the earlier flight, but this is likely to be more complicated than simply checking in separately at ORD. I would check the bag to ORD, and then check in for DEN at the desk after customs (they can issue boarding passes there).