[pxc]

See my other comment and let me know if that script contains

  ssh-agent $SHELL ...

or

  exec $SHELL ...

or

  eval "$SHELL ..."

or similar. :)

If it ends up passing a script file or arguments to $SHELL and you wanna fix it without rewriting it, just add some conditionals where $SHELL is used and use the equivalent flags or syntax when $SHELL indicates fish.

Alternatively, if you just wanna work around it, just launch that script like

  env SHELL=(which zsh) whatever-script.sh