Using a correct-voltage slightly-lower-current power supply is unlikely to actually damage the equipment, but it may cause intermittent operating problems depending on the modem's power draw during various scenarios. After all, if the modem really only needed a maximum of 1.25 A, then why state a 1.5 A requirement on the modem? The main question here is how the power supply will behave when the current draw exceeds its rated maximum (poor rectification, outright shutdown, overheat, ...), which is nearly impossible to know without testing and some test equipment (a pair of multimeters and an oscilloscope will get you a long way), as well as how the modem will respond to that (which is very difficult to know beforehand - some RF equipment, which a cable modem is, can start to self-oscillate when fed unstable voltage, which can cause line disruption or in the worst case a blown-out final amplifier stage).
One important factor to consider is if the modem downloads new firmware over the network and installs it. What if the power supply acts up during that? While it likely won't physically damage the modem, it may cause EEPROM corruption leading to a bricked piece of hardware in a worst-case scenario.
Quite frankly, I would have been more concerned about the technician physically forcing the power plug into the modem receptable. That may have caused physical damage to the relevant circuit boards, which almost certainly is going to come back and bite you at some point.