Diplomatic immunity is an essential protection for diplomats worldwide. If it means that legitimate charges cannot be pursued, sometimes that is a price which has to be paid. However situations like this bring the system into disrepute. Countries can and do waive immunity, and this would seem to be an appropriate situation to do so.

Of course the honourable thing to do would have been for her not to have claimed immunity in the first place.

What especially riles us here is that the US is quick to demand extradition of UK citizens for alleged breaches of US law while refusing to yield up its own citizens. Not quite the same situation, but a close enough parallel.