Resivoir bubbles and. At gets hot

Maybe the new coolant dislodged a bit of rust that was keeping the head gasket from leaking. Try purging air out of the system. Don't blame hubby.

I'm thinking there is some air trapped in the system. If this model has a radiator cap, top up the radiator directly after removing the cap. Then top up the reservoir too. Run it a while and top it up again. Be careful removing the cap on the hot engine. Also examine the cap for damage as the system boils over easily without proper cap pressure. And finally, use a 50/50 antifreeze/water mix and verify the drain plug is not still dripping coolant.