No matter what the reasons are, this introduces a problem of security. You probably should not dump a workstation on a server VLAN. You would probably need to make a new VLAN, perhaps work with the network team, and then fuss with ACLs. I don't know the sensitivity levels of your security, but factoring that complication out, this seems like a reasonable request for a boss with a few potential uses. However, like Raul said, the boss' purpose would be helpful.
The boss would also have a different user experience depending if she or he has direct access to your vSphere environment. Perhaps the goal is to test certain application and runtime versions, such as if something works with just java 7, or if it breaks on java 8, or if both versions of java can run at the same time. Perhaps your boss knows PowerShell and wants to test some commands to see their effect on the domain? What I'm getting at is that each of those things would have the benefit of being able to backstep with snapshots.
If your boss does not have vSphere access, which I'm guessing is the case, then the VM could be used over RDP and therefore function as a thin client. This way your boss can store main documents or install a critical app which can be accessed from anyone else's desktop. Regarding the use of a Windows Server instance for general workstation use, this can be a major inconvenience even if it is grubby with the IOPs and CPU resources. For example, trying to use Windows Server 2016 forces you to whitelist every single website you try to visit, making attempts to browse the web while in an RDP session awkward and cumbersome. This can be worse than it sounds because if trying to download a simple runtime dependency like Adobe then you can expect to need to whitelist over 12 subdomains which run in the background on that site.
Another assumption which might be fair to make is that your boss is simply more comfortable with the UI of Windows 10, even though Server 2016 uses the same kernel. If you are using Windows 2012 R2 and the Windows 8 kernel could very much relate to why you would not want to use that unless you have to. Windows 2008 R2 with the Windows 7 kernel made it much easier (for me at least) to find simple things, instead of needing to fumble around awkwardly trying to get the search icon to appear and in lieu of using any navigation.
I can also see the use of this if a second "computer" is wanted, even if just as a form of extended storage. Perhaps you have a network file share with tight restrictions on it, and users such as your boss don't even have enough room to store a large PST file.
Windows 10, both as 32-bit and 64-bit, is supported by vSphere 6.5 according to the VMware Compatibility Guide (http://www.vmware.com/resources/compatibility/search.php?deviceCategory=software), so there would be no technical problems aside from the points about efficiency.
