We have a special needs adult daughter, in a shared living arrangement with roommates, who was ALWAYS kind, sweet, and very cooperative. We'd never heard any negative commentary regarding "Sally", prior to an unknown roommate arriving on the scene. "Daisy", a higher functioning girl, from an unfortunate background, easily charmed both of us, while possessing a measured, devious character, and being very persuasive, literally taught & modeled for Sally, that it wasn't necessary to do ANYTHING staff asked, including basic hygiene and chores. Before that time, we'd never received a hint of a problem. Since then, our Sally has increasingly been a "stinker" to her staff and caregivers. What is a good noun for such a person, who "demonstrates" for an innocent the "ways of the world"?
With respect, I encourage consideration of a more nuanced appellation. The genteel assignment is “unfortunate influence.” However, simply branding the ill mannered girl will do little to aid and assist your daughter’s happy and peaceful enjoyment of life free of the burden of someone else’s boorish sport degrading her behavior.
Using the term to firmly inform upper level staff and management, or your fellow associated guardians, of the unacceptability of the current arrangements. To describe the offending interloper’s discreditable influence to those in a position of authority and responsibility to rightly require their intervention is the way in which whatever appellation you ultimately find to be appropriate will have a desirable outcome.
corrupter / corruptor (n.)
One who or that which corrupts: in various senses. OED
To change from good to bad in morals, manners, or actions m-w
He was a corrupter of virtue, an habitual drunkard and gamester, a notorious blasphemer and freethinker, a fitting companion for my Lord March, finally, and the company into whose society he had fallen. William Makespeace Thackeray; The Virginians
Xenophon specifies that Mausolus, not happy about abandoning his camp, gave Agesilaus a large sum or money with no apparent compensation. The Spartan was thus described as a corruptor who not only did not give gifts, but received them. Vincent Azoulay; Xenophon and the Graces of Power
Other rulers had set an equally bad example at the beginning of the century. In July, 1706, Duke Louis of Wurtemberg married, as an additional wife, his courtesan, Gravenitz, the “corrupter of the country,” as she is still called in Wurtemberg. August Bebel; Woman and Socialism