I am not sure if there is an example of that specific situation as it would be pretty odd for a phone to be receiving security patches but not firmware updates.
Anyway its not super relevant as the Pixel 5 does not receive firmware or security patches anymore.
OP also seems to be inferring he suggested to his friend to use a very specific security / privacy OS that does not recommend using that model phone anymore for the exact reasons I mentioned. Plus the model is only receiving partial support as a stop gap for users to have time to get a newer model and won't be supported much longer anyway.
Not sure where your getting your information but the Pixel 5 has not gotten Android updates or security updates in over 7 months.
There are tons of examples of exploits being used to target EOL phones as its common for people to not care about these updates, or be misinformed, so they are easy targets.
If OP or anyone else wants to use an EOL phone that's fine but, don't pretend its a smart security practice. Although even if I were to use an EOL phone, LineageOS doesn't have the greatest background and isn't really degoogled