Interpersonal conflict can arise between any people.
If a player has an issue with another player within a gaming group for a game that has a DM/GM/Keeper, what responsibility does a DM have to help resolve interpersonal issues?
Interpersonal conflict can arise between any people.
If a player has an issue with another player within a gaming group for a game that has a DM/GM/Keeper, what responsibility does a DM have to help resolve interpersonal issues?
Generally, none at all, or at least no more than anyone else at the table.
This depends on the social contract in place at the table in question. But most social contracts are informal, rather than something you actually write out. Most RPG groups are groups of friends or acquaintances that get together to play the game. In that situation, the GM generally has little to no role in resolving interpersonal conflicts, at least as the GM. In most situations, when it comes to interpersonal-conflicts, the GM is just one more acquaintance or friend in a circle of acquaintances/friends that happen to participate in a hobby together. Any action the GM takes to try to handle interpersonal conflicts is simply as one of many acquaintances or friends.
Notably, in many groups, GM is a revolving position and doesn't imply any more power or influence than the rules themselves give and that power exists only within the game world and comes with responsibility for helping ensure everyone has fun.
Of course, this can vary. Some groups, especially if they are only acquaintances that only or primarily interact during the game, may actually write up a social contract. They could appoint someone to handle conflicts. It could be natural, depending on that group, for the person appointed to be the GM. Of course, the two roles don't need to be merged, especially if "GM" is rotating position.
Also, if you are playing in a public place like a game store that provides space, there is probably one designated point of contact between the store and the group. At least from the store's perspective, that point of contact is the person responsible for at least keeping any interpersonal conflicts civil and quiet. That person might stereotypically be the GM, but there is no reason it has to be.
These roles - DM/GM/Keeper or any other 'administrator' - are titles within the game system. Being a Wizard doesn't mean the player can throw Fireballs, and being the DM doesn't convey any special privileges or responsibilities outside the game.