The main point of this sentence is that the speaker had just had dinner with his close friend Sostratos--and, as he goes on to say, could have asked him (Sostratos) to stand by as he killed Eratosthenes if he (the speaker) had laid a trap for Eratosthenes to settle some lesser account unrelated to the seduction of his wife (which, of course, wouldn't justify killing him). (Instead, he lets S. leave, and only after he discovers E. in flaganti goes to round up the neighbors, or so he says. Personally, I can't say that this line of reasoning is wholly convincing, but a lot of arguments offered by the Attic orators invite skepticism.) That's why ἐμοὶ is very emphatically placed at the beginning of the sentence. The other words and phrases -- φίλος ὢν, οἰκείως διακείμενος, and ἀπαντήσας -- are both grammatically and semantically subordinate to the main point, i.e., that the speaker had just had dinner with Sostratos, who would have been available as a back-up if the speaker had been planning to murder Eratosthenes.
In Greek, an explicit complement to these subordinate ideas repeating the pronominal complement of the main verb would be not just superfluous but otiose. For example, if instead of ἀπαντήσας, the speaker had chosen a participle taking an accusative complement, such as ἰδών, or genitive, such as τυχών, there would be no need in Greek for an accusative pronoun με or genitive pronoun μου, in addition to ἐμοὶ. Similarly, if the main verb were one that required an accusative complement, there would be no need to add μοι as a dative complement to φίλος ὢν, οἰκείως διακείμενος, and ἀπαντήσας. And adding μοι would be not only unneccessary but excessive in Greek, although in English we would need personal pronouns to go with these ideas. This is a difference between Greek and English.
So I think that purely as a grammatical question, ἐμοὶ is the complement of συνεδείπνει, although, of course, ἐμοὶ confirms that the speaker is talking about himself when he says φίλος ὢν, οἰκείως διακείμενος, and ἀπαντήσας.