Answer is simple and the one that your customers would like to hear!
YES – because the underlying physical server hardware is dedicated to the end-customer, partner can provide Windows Server and RDS (under SPLA) alongside O365.
The main rule here is that both parties (i.e. service provider and the end customer) must not provide licenses that belong to the same product family.
E.g. Windows Server and RDS (via SPLA) + Exchange/ Office (end customer license/O365) = ALLOWED
RDS for 10 users (via SPLA) + RDS for 5 users (end customer license) = NOT ALLOWED