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.
I’ll also venture then that Gregory of Nyssa may have been a saint, but was also wrong, or misquoted. You seem to be quoting from wikipedia.
Here it is from Saint Matthew: “Lay not up for yourselves treasures upon earth, where moth and rust doth corrupt, and where thieves break through and steal: But lay up for yourselves treasures in heaven, where neither moth nor rust doth corrupt, and where thieves do not break through nor steal: For where your treasure is, there will your heart be also. No man can serve two masters: for either he will hate the one, and love the other; or else he will hold to the one, and despise the other. Ye cannot serve God and mammon.”
Mammon is cognate to the English word money. Some people, even today, make money their false god. But it doesn’t make money bad or good; it’s just money. How the person gets it, or spends it, makes that person bad or good.
If this troubled woman believes the mark has a demonic origin, I agree, she should be in contact with her church. Or, if she doesn’t have one, she can be in contact with the office of exorcism in either the Anglican or Catholic Church in her area.
This is a story from a Malaysian news source, oddly. I’d think a more understandable mark of mammon in England would be a capital L in script with a line through it, rather than something that looks like many Christian crosses even in the un-Celtic country of England.