@5gbps.com, Client is allowed to use own license if the dedicated server is entirely used by him. It's up to the client what he wants to use (i.e, trial or development etc.) However, if MS finds the illegal usage of their product, they have full right to seize the server regardless of the ownership and can take the client to court.
Also, there is a common mistake by certain VPS providers that we don't have any SPLA agreement so we just provide the Windows ISO/Template and client needs to provide license for Windows OS. Unfortunately this is completely illegal. The VPS provider rent server from DC and sells VPS. In this case this a commercial activity and VPS provider is required to take SPLA License key from the DC or become MS Authorized SPLA partner.