You are making an assumption that all bad breath is caused by mouth bacteria (Listerine is mainly effective for bad breath caused by bacteria in mouth/tooth).
BUT, bad breath can also come from the lungs.
When colon, liver and kidneys are overloaded with toxins, and especially when harmful bacteria and yeast gets a hold inside colon, air comeing from the lungs may smell very bad.
Now, this is of course not an excuse for that gay, because it is not only showing that he is bad with mouth higiene, but he is also bad with his inner body higiene.
You should have actually suggested him to try CureZone.com instead of Listerine.
One good enema or colonic with added probiotics could make his breath perfect, at least for the evening.