Don't rule out the fact that the smaller one was sick anyway. I've had experience with a pair for 3 years. When one got sick, the other would harass the other to no end. Took the smaller one/harasee (new word) out and treated it. When it was well, I returned it to the main and the larger one ran to great it and they were fine for the next 4 years. Not unusual to have a fish know another is sick LONG before you know.
Do not punish the remaining fish!! She was just doing what fish do. Try a new, much smaller mate.
"Life may not be the party we hoped for but while we are here we might as well dance!"