Please take this in the spirit in which I offer it, as sound advice for the well-being of your fish, and to avoid disappointment to your daughter when this plays out.
First, all loaches are highly social fish. They must have a group, and I would never consider fewer than five of the species; with fewer than this, subordinate fish of the group will be harassed and other fish in the aquarium may be attacked particularly as the fish matures. This fish develops an hierarchy within the group once they are together, and usually (perhaps always, this has slipped my mind at the moment) a female is the alpha fish. Other subordinate fish will regularly interact with the alpha, and each other too, to maintain their hierarchy. This is not only absolutely essential for the health and well-being of the fish, it also provides the aquarist with some very interesting enjoyment.
Tank size is another issue; this fish, in a group of five, needs at least a 6-foot tank. It will attain 12 inches on average, though some report 16 inches; between 8 and 12 inches is normal in an aquarium. Growth from the 2 inch size often available in shops to 5 inches is quite rapid so the initial aquarium still needs to be large.
The above needs are programmed into the genetic makeup of this species. The fish "expects" these things, and without them it will not be in good condition, which means stress leading to further issues. I cannot tell you what to do with your fish, but I would suggest that under the circumstances, returning the lone loach would probably be the best course, and soon. There are other loach species that are significantly better for home aquaria, as they remain in the 2, 3 or 4 inch mature size depending upon species. All need a group.
