I found this shaila so interesting that I asked the original question to my own rav. He said that while he wouldn’t tell someone to daven to the Rebbe, we can be dan the woman lekaf zechus and assume she was just asking the Rebbe to be a meilitz yosher to Hashem, not that ch”v the Rebbe runs the world instead of Hashem. So this person can be thanked, as it’s not pashut they did an aveirah. However, if someone did a vadai aveirah, they can’t be thanked. Years ago, I helped a feledgeling Orthodox shul that was located in the basement of a Reform Temple (yes, the shailos were asked in it was cleared by poskim). I went to use the facilities. A Reform kid came out of the bathroom and shut the lights. He saw me coming and turned them back on. I have no right to thank him. And when you hold a tinok up to the light and he, on his/her own volition, turns it off/on, you are not allowed to show any sign of happiness to the kid.
