The New York Mets were leading the Philadelphia Phillies, 2-1, after two innings when Tom McDonald stood up from his upper-deck seat at Citi Field.

Nature was calling, and so was his obligation to his childhood friend and fellow Mets fan Roy Riegel, whose death nine years ago left Mr. McDonald, 56, vowing to honor their baseball bonds in an unconventional way: by disposing of Mr. Riegel’s ashes in ballparks across the country.

Even more unusual was his chosen method: flushing them down public restroom toilets in the ballparks between innings.

“The game has to be in progress — that’s a rule of mine,” Mr. McDonald said one recent weeknight before entering a Citi Field bathroom, holding a little plastic bottle containing a scoopful of Mr. Riegel’s cremains.