it's not stupid. it's not like we're telling him to use it for kindling. assuming he's in his 30's, he has plenty of time to save more for retirement and invest that money. it's the same with paying off your mortgage when rates are this low. if you sleep better at night, do it. if not, great, invest it instead. there's no right or wrong answer.
that's exactly what you're telling him to do with it. you're not telling him to the use the money for something else. he's already set the parameters - he wants 2.5 months in reserve for the rest of his life for whatever reason. the question is just whether to let the money make money while it waits for an emergency, or whether to have it hunker down at the local bank and depreciate out of paranoia of having to use it at some point when the market is down, or when you need it within 72 hours or god knows what the logic is.
if you assume even highly conservative returns, we're talking about a difference of hundreds of thousands of dollars for 80 year old kk. it's not a coin flip, either decision is right question. these are highly divergent results.