No cheat !
That was another guy , an usual customer of theirs that showed up a couple of times at the restaurant and was kinda into shinobu but in the end shinobu didn't take account of his feelings and just focused on tatsumi San (=・ω・=)
In the end the manager was dwelling on his broken heart and so was the guy who was after shinobu
So they were both teasing each other for their unrequited loves and they got laid in the spur of the moment and with the help of alcohol ofc lol.
the mangaka suffers from "same face syndrome" which is why Shiraichi-san (the regular customer of the restaurant) and Tatsumi-san (Shinobu's bae) and actually, Shinobu himself, look an awful lot alike.
Don't get me wrong, they're gorgeous and the art style is beautiful but there's not much variance between characters which is a problem and many other mangaka are guilty of it.
So in the extra the guy with the black hair is suppose to be with shinobo right? So why he was in a bed with the manager or was that him or not??? Did he cheat ?