Horizontal timing-wise, if the x-tal is a good freq standard, the scope is well within cal. The manual specifies the 7904 horizontal accuracy to be 0.1div or better. Mine looks like 0.05 or 0.06, so it's good. I didn't need to adjust much of anything even though this 7904 spent 20 years on the shelf of an e-recycle center, got rained on several times (because the recycle center likes to leave some stuff outside), and the fact corrosion is everywhere inside this thing.
The power supply is in spec, horizontal is in spec. Horizontal is in spec, (timebase plug-in only needs to be adjusted), vertical is mostly is spec, z-axis is in spec. Calibrator output is a bit off, but that was adjusted.
These were checked with the Global Specialties 4010, Tek PG501, Tek FG501, Tek 191, Tek 2230, and Tek DC503. All these have been calibrated recently (2019) by my local college, because that's where I got them from.
I have since cleaned up the scope very nicely, removing most of the corrosion.
I would also like to provide an update that my friend has accepted defeat. But he says he'll still use his two rubidium frequency standards to calibrate his scopes, averaging out the two frequencies and finding "sweet" spots.