Swapnil,
This would be of high importance if it stops my basic business proposition to my customer or client. At this stage, it isn't.
Like I said, I would probe into it. In my organization, we are very disciplined. We don't entertain people coming in for meetings without being prepared for the agenda (and I mean real well prepared). We also don't like people taking rules too casually. And we never allow people to talk through their hats when discussing work!
The newbie is probably right about the behavior, and apparently, and I never denied corrective action. But is it such an issue? I don't think so. From what I gather, this is some software company and not a skill development center.
The Hiring Manager, for me, is someone who has committed to provide me with a resource on a specific date. If he hasn't got the guy joined, he has failed his target. So, his pressing on the formalities is understandable. At the same time, if the person wanting to join is not prepared for it, he is also not appreciating his 'fitment criteria' in the organization.
There is no need to overreact. Its strictly business.
Now coming to bad conduct. At times people are sensitive. Hence, it would be better to talk it out to the manager and see. If it DOESN'T GET RESOLVED, only then involve the stakeholders. For I would first call both the guys and hear their stories. If the issue isn't resolved, then I might call for a formal inquiry. But before I take any action, my first question would be to the newbie: Did you tell him about this? What was the reaction? What do you expect the reaction to be (assuming it was unsatisfactory)? And after these questions, I would look at the plan of action given above.
Remember, as HR, you need to understand your business perspective. Its easy to fire people, and easy to hire as well. But it makes sense to understand the business driver before we take drastic measures.
Hope I answered your question dost.