Well, he is her co-worker, so I don't see a problem with exchanging phone numbers. Of course with this being H!P and MM, the normal rules don't apply.
It was pretty obvious since the beginning Aka-chin liked Ai-chan. He always let her win and talked to her the most. I can't believe management didn't stop the romance before it got out of hand (read: splashed in tabloids). I just hope she doesn't have to graduate so soon.
If the graduation happens next year it won't be a surprise since most years have at least one graduation. But if it's this year, there's a high chance the rumor is true *sigh*