@Italiangreyhound
I don't work in HR. I'm the boss of our organisation. We have a largish personnel dept. but very small HR. We have legal teams on retainers should we be unsure about a process but on the whole, I sack and discipline. 80% of my staff have post-grads so there's rarely any discipline necessary. Staff are on rolling 3 year contracts so underperformance usually means non-renewal although if we think it worth it, we would of course help them improve as part of on-going PD.
Regarding othe rpoints:
interrupted her on a break
He told her he'd brought her something up. In my work place we get breaks but often use the opportunity to pass on info. Breaks are frequently or sometimes dropped or shortened if necessary. I don't think this interruption is a problem.
was sarcastic in his tone
I've just said "I've brought you your supplies" out loud a few times and can't do it sarcastically. Maybe he's responding to the fact the OP and her cronies 'tolerate' him. He came to find her and let her know. This is a small way of making extra effort that I would encourage or expect from my staff.
swatted him with a newspaper
This is unacceptable. The point is not whether it hurt or not. Considering the strained relationship with them, she was an idiot for doing so. If I saw a member of staff doing this and knew there was some kind of flirting, I'd still give them a slightly raised eyebrow look.
There is no evidence she is a manager or a manager of him
Did I make that up? Maybe! Perhaps it was the way she said "who do you think you're talking to" or has it been mentioned up-thread? Saying "who do you think you're talking to?" suggests she is superior to him in the organisation or at least thinks she is. It certainly isn't a pleasant way to conduct yourself. If she isn't the manager then I'd be questioning her professionalism.
Ignoring the retaliation, which you would do when deciding if the OP had been professional, it's clear to me that no, she hadn't. She sounds fairly immature and needs to be reminded to grow up. If she is a manager then it would be more serious as it's such a lack of judgement I would be investigating to see if it was a one off or pointed to other aspects of her work and behaviour.
Of course he shouldn't have hit back and from the sound of it, hit her harder. He should be dealt with. How depends on how hard he hit her ie. leaving a mark and what type of mark. He should have walked away with legitimate grounds for a complaint. Not one of assault but unprofessionalism.