Binding Theory blocks any interpretation of this sentence except that in which "he" refers to "One online friend...". Neither Breck nor Daynes can be the antecedent of "he" because they are too deeply embedded and fail to c-command "he". On the other hand, non-restrictive relative clauses, contrary to restrictive ones, cannot be iterated and so must be adjacent to the nominal they add information about, i.e., the antecedent of "who" must be Daynes here, and it is Daynes that is described as a 'control freak'.
You can get the alternative interpretation you want (i.e., the 'control freak' is Breck) simply by placing a restrictive relative clause directly after "Breck", as follows: "An online friend of Breck's who described him as a 'control freak' claimed that the teenager had been groomed by Daynes."
The sentence is a bit strange because that adverse characterization of Breck seems a bit difficult to relate to what the sentence seems to intend to say, i.e., something about Daynes and the teenager (= that it was Daynes that groomed the teenager), not anything about Breck's character, but, grammatically speaking, in that restrictive relative clause "who" must refer to the referent of "An online friend of Breck's" and "him" cannot refer to Breck's friend and must refer either to Breck himself or to a third individual identifiable in the co(n)text. Since no co-text or extralinguistic context is provided here, just your sentence in isolation, by default "him" will be interpreted as referring to Breck, as you wanted.
This, by the way, also follows from Binding Theory and from the syntax of noun phrases: since "who" c-commands "him", by Principle B it cannot be its antecedent, they cannot refer to the same individual. On the other hand, since "of Breck's" is a relational argument (a complement) of "friend", it must be attached to "friend" as a sister, and lower than the restrictive relative clause, which is a modifier (an adjunct), not a complement. As a consequence, "Breck", buried inside a PP complement of "friend" does not 'c-command', and cannot be the antecedent of, "who", which is structurally higher. Since only the projection of "friend" is a sister to the restrictive relative clause, only " online friend of Breck's" can function as the antecedent of the relative pronominal "who" and so it is Breck's friend that describes Breck as a 'control freak', as you demanded.