My friend had a toilet that he replaced the wax.For two years you could smell sewer stench.
Just reacently he asked me to replace the wax again because I seen the toilet puring water into the bacement.When I left the toilet only half the wax (if that) was there.I cleaned the entire toilet inside and out.I then replaced the wax reset the toilet and PUFF no more smell or water bath in bacement.Now he said when there tub drains the toilet gurgles.

Sounds like your friend has problem with the vent and needs to be snaked from the roof, run enough snake down the vent to reach roof to basement floor plus another 10-15 feet.