The tamiflu does nothing to the parvo, as parvo does not use a neuraminidase, the enzyme that tamiflu inhibits. It is speculated that it likely affects neuraminidase of bacteria involved in the secondary sepsis. Tamiflu resistance is a problem with influenza viruses, but not for parvo.
There has been quite a bit of debate about Tamiflu use in parvo or parvo exposed patients. Right now there isn't enough evidence to make this a recommended treatment, although many try it and have variable success.
Even if Tamiflu helped with inhibiting parvovirus replication or the secondary bacterial sepsis, there are still serious systemic problems that need to be addressed with pretty intensive therapy (IVs antibiotics, plasma, antiemetics to name a few)..