You will want to add her name to reservation regardless. If you are separated and she loses her key, the only way for her to get into the room is if she is listed on the reservation.

As for the rest of the scenario, what you describe is not standard operating procedure. That fact doesn't mean it's not possible. But, it certainly does increase the possibility that something might go wrong.

For this to work, you will need people in multiple departments to agree: Front Desk, Reservations, Housekeeping, and potentially Accounting. You may even need some rules bent or broken. (Example: Front Desk will need to check your reservation into a room that isn't clean and vacant.)

In the hotels in which I worked, we probably would have worked this out. However, I can easily picture a hotel manager saying "no" because she or he doesn't have extra time. Or, simply because she or he is a stickler for the rules.

If the situation is allowed, just make sure that your reservation has your HHonors information attached and that you pay with your credit/debit card. That should be enough to assure that you get the stay credit and points. The fact that your friend stayed on a points booking shouldn't affect your cash reservation.