Say a member of the party has had Suggestion cast on them by a hostile creature.
Can the other party members end the spell by attacking the affected party member?
Or is that condition for the spell ending restricted only to the hostile spellcaster and their allies?
This is spelled out explicitly in the spell description of suggestion:
If you or any of your companions damage the target, the spell ends.
This is the end condition. “Companions” is defined in relation to the caster. Since your allies are not companions of the hostile creature, they cannot end the spell by damaging you.
This ruling should cover most situations, but this question goes into greater detail about how we are to understand what a companion is, and may help if ever it is not clear.
