He told me that he was treated very friendly

Explanation

This is the error. The word "friendly" is not an adverb, but an adjective. So in order to describe a verb (treat), an adjective "friendlily" should be used. The word "friendly" should be modified to "friendlily" to match the adverbial use in the sentence. Adverbs typically end in -ly, and in this context, "very friendlily" is the correct form. Don't confuse suffix -ly of friendly with it being an adverb it is not! So, the corrected sentence is: "He told me that he was treated very friendlily."