I’m pretty sure my 81-year-old mother would have done the same. A bed, or any part of it, isn’t exactly the proverbial Immovable Object.
My son and I recently put his bed back together in his refurbished room. Since I’ve had back surgeries, I let him do much of the heavy lifting.
To put in the slats after the fact (basically same idea as @Galactus) we moved the bed onto one of the bed frame side rails, sliding the box spring / mattress combo along. A piece of scrap wood acting as a crowbar can help shove these things along, including getting the bottom edge of the box spring back up and onto the bed rail in the first place.
Then, let the whole balance–another person can lightly hold it in place–and one by one, put the slats back into place, now that you’ve got room to work. Then reverse the process, gently sliding the stack back into place, again using the stick for extra leverage.
This is how I did it.
I feel compelled to add that the room in question is somewhat cramped and the whole “slide to the side” thing would not be practical.
So, filling the room with water is making more and more sense