Can I drill holes in septic tank risers to off-gas hydrogen sulfide?
Both tanks (in fact, the whole system) should be designed to vent gases back through the house vent stack. Drilling holes in the "riser" (are you calling the inspection port/manhole the riser?) will release the gas there and cause odor at that area. It could also expose an innocent passer-by to poisonous H2S gas.

I would be careful following this guy's advice. If the system required additional venting it would have been designed and installed to provide that.