Usually in these cases it turns out the recipient is not actually the RK - i.e. the car is on lease / PCP or similar, and the finance Co is the RK. So, is the car leased / does your partner actually have the V5C in their possession, and is the address on it correct (don't assume, check!)?
However, as you say, the named recipient of the NIP must respond naming the driver, and you will then get one in your name - to which you must also respond. At this point, assuming that car is not leased etc, and V5C details correct, you can point out that the NIP was late. It may get dropped at that point, or you may be able to defend it in court on the basis that it was not served in time - or you may consider that taking the Conditional Offer (if you can) of a Speed course or 3 pts / £100 is the easier option.
