HR has a vested interest in making sure that the M&A run smoothly and as transparent as possible. At the the end of the day, they will get the employees to onboard and manage. HR should be involved very early in the process to me, from the target screening as much as possible.
I found the following publication of interest and tied to the topic you raised: The HR Practitioner’s Guide to M&A (Due Diligence) by Klint C. Kendrick.
HR is one of the underrated areas in due diligence, the key element in a business’s success is the people. The critical thing is to make sure that the right people with right capabilities are preserved during M&A.