Plants in hospital tank?

I put my male betta in a 5 gallon hospital tank yesterday. He doesn't seem
to have any sort of disease, he just looks worn out, lazy, and not eating
much, so I'm not treating him with anything, but I'm hoping he'll have an
easier time eating alone. Will he be happier if I put in a spare hygrophila
plant I have (it's floating because there are roots coming out of
everywhere)? He has an established sponge filter and a heater.

Martin