Bizarre. Especially considering that mammon is not actually a devil or descended from a devil, or even, The Devil. And also bizarre considering that the mammon mentioned is ‘the mammon of unrighteousness’, which points to there being another kind of mammon. I don’t know what made the Celtic cross mark on her little boy, but I’m willing to hazard that the explanation is more mundane than the internet is helping her with.
Early mentions of Mammon appear to stem from the personification in the Gospels, e.g., Didascalia, "Do solo Mammona cogitant, quorum Deus est sacculus"; and Saint Augustine, "Lucrum Punice Mammon dicitur" (Serm. on Mt., ii). Gregory of Nyssa also asserted that Mammon was another name for Beelzebub. In the Bible, Mammon is personified in Luke 16:13, and Matthew 6:24, the latter verse repeating Luke 16:13. In the Greek, Luke 16:9, and Luke 16:11 also personify Mammon.