I read from 245a slowly & carefully with the help of the English translation @Perseus,
but I hope I'm not misleading you.
In the previous sentence you had:
πεπονθός τε γὰρ τὸ ὂν ἓν εἶναί πως οὐ ταὐτὸν ὂν τῷ ἑνὶ φανεῖται,
καὶ πλέονα δὴ τὰ πάντα ἑνὸς ἔσται.
The underlined participle means having had unity acted/imposed [upon it], referring to τὸ ὂν, being, as the subject.
Then, the stranger adds:
καὶ μὴν ἐάν γε τὸ ὂν ᾖ μὴ ὅλον διὰ τὸ πεπονθέναι τὸ ὑπ᾽ ἐκείνου πάθος, ᾖ δὲ αὐτὸ τὸ ὅλον,
ἐνδεὲς τὸ ὂν ἑαυτοῦ συμβαίνει.
Now the participle became articular infinitive with object πάθος and, since the verb has passive meaning, passive agent, ὑπὸ τοῦ ἑνός.
I'm assuming it's ἐκείνου because it goes further to his previous statement.
So here the underlined part means because of the attribute having been imposed [upon it] by unity.