If NR dies on Pi1, that doesn't mean that a ping won't work. If Ping doesn't work, then Pi1 could be hung or it could have crashed altogether and only a power off/on 'might' fix it.
To do that, you would need a relay (maybe a smart plug) that Pi2 could send a OFF/ON command to causing Pi1 to reboot.
If NR is not responsive but Ping IS working, then you could try having an EXEC node use an SSH command to restart NR on the Pi that is not working (haven't tried this myself)
You could also try one of the ssh nodes (search in the flows tab)
and this thread might be of interest Run SSH commands from within a flow
