I would just work with the newer live rock, how long was it not in the water totally? Did he have it wrapped in newspaper or anything?
What I would do is this: Put everything back in the 75 that came with it. Did it have sand too by the way? Anyways, have everything in the 75g fill with salt water and let it go. I would test everyday to see if there is a spike of ammonia or anything else. It could take a few days or a week or two I would think tops. The rock you got, even if it's bone dry will still have enough life left in it that it should bounce right back.
You do have an API testing kit right? For ammonia, trites and trates?