Is HR allowed to veto a hiring decision by management?
Simply put, generally, no but there are various scenarios and circumstances. So this is really not a yes or no question and requires more information to answer.
Legally any Officer of the company (a legal status according the Secretary of State filings in the state in which your company is incorporated) has the right to commit their company legally to any contract, and hence to hire someone. This status comes with a “Fiduciary responsibility” to act in the best interest of the company and its stockholders. Of course, if that Officer is also violating clear company policies that may not stand. Or if that Officer has a superior, who is presumably also an Officer, they would have the right to override that decision. Of course, the President/CEO has the authority to override virtually any decision too.
The reality is this should never really happen in a good culture. Any Manager should generally have full discretion in hiring decisions. If there is an HR Department, they should facilitate that process but have little legal authority really. However, companies can set any policies which their managers must abide by too. So there is no clear answer without a full review of this policy manual and all facts. If this is happening it is a symptom of other problems and discord within the culture that should be resolved.
Professionals deal with these disputes internally, quietly and privately under high levels of confidentiality to not upset others and show alignment of the management team. Generally disputes like this should never go public, even internally inside the company. Any manager doing so is acting unprofessionally and does not understand their responsibilities very well. A good one would honor the policies, or seek a waive/exception from someone with superior authority over both HR and them. Not doing that is a symptom of other problems in the culture and management team.
Good management teams act collaboratively and should rarely, and only behind closed doors, air disputes. This requires understanding of the bigger picture, maturity, self control and a team spirit. If this kind of internal conflict is happening your company likely needs either a Management Development (MD) program for ongoing training, or an Organizational Development (OD/OCM) program to address the cultural problems implied by this issue. See information on these here: http://www.airtightgrowth.com/articles/organizational-development-program/
Bob Norton has been a CEO since 1989, grown two startups to over $100 million and trained and/or coached over 1,000 CEOs in 35 countries. He is also the creator of AirTight Management, The CEO Boot Camp and a thought leader in entrepreneurship and scaling expert. See http://www.AirTightGrowth.com