You mean the temperature of the tank is 78, right? If thats the case, that will be fine for them. :)
Sick fish, huh? Sick with what, do you know?
Either way, an interesting "cure-all" for betta fish seems to be Epsom salt. If he's really sick (or on deathbed) try 3tsp per gallon. If he's not so sick, 1.5-2tsp would work fine as well.
Sick bettas typically need warmer water to help them heal or get better, but 78 degrees is fine for even a sick fish.
Trust me on that- I've brought a fish basically back from the dead and his water wasn't even as warm as 78 (I was on vacation for two weeks and had to leave him. The epsom salts worked well).