I'd add to this that it's only effective monitoring if he has a gauge installed (or wifi dongle for phone). Otherwise the temp needle won't move unless it's starting to overheat. If you hit that point, clearly there's a problem... but it could be running warmer than it should due to gunked...
Hopefully that was before their work!
Regardless, to start, easy to do a combustion gas test on the rad to see if the coolant is getting contaminated. Fluid neglect/mixed coolant isn't "good," but better than a HG..