Is it acceptable to run LV conductors in the same conduit with the regular voltage conductors. For instance, undercabinet lighting with the xfmrs in the basement, run to a kitchen above in EMT with the 120v conductors.
I don't think anything is wrong with it, provided the insulation is rated for the highest voltage on all the conductors.
Sounds like he is putting the ballast for a light fixture in the basement. Check the xfmrs secondary output voltage. It may be higher than you think. Of course, if the xfmr is truly a step down version, then as pointed out by Joe Tedesco, all the conductors in the same raceway, enclosure, or cable must be insulated for the highest voltage.