I've considered that, adding a 400 watt MH.
my highs are 78 which is when my flower room lights are on and my low is 70 obviously when flower room lights off. So what I did was turn down intake and exhaust fan and that should raise things enough. I will probably upgrade the electrical soon so I can add a heater. This is a new room and as always with new rooms, they need to be dialed in. I just checked the girls today and they are looking better.
They'll be fine. Those temps aren't a problem. If the old growth gets crispy, prune it. But things should get vigorous again in a couple days.
Once the plant starts grooving again, give a light DeStress Spray. Hold off on that now, as it's obvious the soil isn't wide awake and the spray won't have the right result.