I attacked my friend and his planeswalker, who had a Palace Guard in play. Now can his card block for himself and his planeswalker at the same time? Or would he have to choose which one it blocked for, meaning himself or the planeswalker?
When a player declares blockers, they declare which attacking creatures they will block, not which planeswalker or player they will defend. From the official rules:
509.1a The defending player chooses which creatures they control, if any, will block. The chosen creatures must be untapped. For each of the chosen creatures, the defending player chooses one creature for it to block that’s attacking that player or a planeswalker they control.
Palace Guard has an exception written on its card that it may block any number of creatures instead of just one, but there are no rules that forbid a card from blocking for multiple attacking 'targets' simultaneously.