This article's content is accurate to the most recent public game version, 0.4.8d.
Wendy's Help is a status effect that grants the user immunity to several status effects that have to do with the mind. These effects are: all forms of 
Charmed, 
Confused, and 
Stunned.
The only location Wendy's Help appears is in the Hallucination dungeon, granted to Eric(a) by finding and speaking to Wendy within the area. This buff will greatly help throughout the dungeon and against the Magico at the end, Bane of Sanity.