Raccoons climb, and are nocturnal. I don't know is opossums would climb to a 2nd floor patio, but raccoons would.

I wouldn't be surprised if an oppossum could climb that. It's probably a toss-up between coons and oppossums. Look around the outside of the deck, and down on the ground level, and see if you can see any scratches on the wood. I'm always finding coon scratches on my chicken coop door, and wooden 4x4 posts on my fence in the back yard.
I just re-read your post and see that there is a set of stairs. Have her look for paw prints on the steps and you might be able to i.d. the animal that way. She could put some flour or something like that on a step or 2, and then check it in the morning.
I doubt rabbits would climb that high, and groundhogs aren't noctural.
Maybe your mom could put a cheap gate up somewhere along the steps and it might deter them.....but I doubt it. Especially now that the animal knows there's a buffet up there! Keep us posted!

I have squirrels that eat the tops of some plants and eat others all the way to the ground (Perennials and shrubs). At our last house, I bought an inexpensive baby gate to keep our dog from going down. It never stopped coons or squirrels. I would do what Catherine suggests, but I also wouldn't rule out the dang squirrels.

The squirrels dig in my flower beds and even 'steal' plants and such. I had lamb's ears come up a hundred or so feet away from where I planted them and I suspect the squirrels of carrying them away. No way they washed that far away from where I had them. Oak trees sprout in my mulch. I find their little holes all thru the beds. I am pretty sure they took two whole lillies this spring as they just disappeared completely. Wonder if they will come up next year with the lamb's ears.

Kiki

Don't mean to crash your thread, but since I had just posted on the squirrel issue I had to tell you this. I had been suspecting that some of the seeds that were sprouting in one flower bed might not be a flower. I told you about the oak trees....sure enough my husband confirmed that these are indeed potato plants as I suspected. He told me that he had dumped a bucket of old potatoes in the woods. I guess the squirrely guys couldn't resist burying some of them in my mulch pile, lol.
Kiki

I have seen many oppossums in trees eating bird seed.

are you positive this is happening at night? Although I will bet your thief is a squirrel, groundhogs can and do climb trees when they want and depending on how your railings/steps are finished and what all you have growing that might be tempting them, you can't rule them out completely. Plant some young, tender soybeans up there and you would likely see chuckers sprout wings and fly!

Sounds like more sleuthing is in order.....

Try putting a coating of flour on the deck around the planters. When the critter comes back, he'll leave his footprints! Then just look up the tracks to find out what it is.