A friend of mine has started preparing a pbp Mutants & Masterminds campaign, and one of their players is someone I know to be problematic.
Already, the player has started to show signs of being troublesome for the DM's campaign. The player has requested excessive help in designing their character - from requesting guidance in coming up with their own character's powers, to asking for personal one-on-one RPs with the DM to flesh out the unfinished character concept.
As an example, the player has asked in group-chat what their character should be like as a cat. The DM informed them it could be anything they wanted, which they player said was 'unhelpful'. When the DM and other players offered the chance to go over the mechanical aspects of it, the player refused to do so, 'humorously' saying "Too hard, let's move on ;p".
This player has been problematic in campaigns I've been in before. In a Truth & Justice pbp campaign with a different DM, the player regularly took over a week to post their character's actions, by which point other players had to start describing what their characters were going to do. The player then got upset at the DM, accused the DM of being a bad friend, and abruptly left the campaign.
This spilled over into another pbp campaign the same DM was running - a Maids: RPG campaign, which the problematic player also left because of this conflict with the DM. The rest of the players carried on with the campaign without this problem player, but it greatly impacted my own characters, because I'd gone out of my way to make my own characters someone this problem player could interact with.
This player can be creative when pressed for it, and can be fun to play with, but they require a lot of attention, both from the DM and from the other players. They're also very sensitive, and react harshly to any perceived slight against them, but don't see their own behavior as problematic. I am concerned that any future campaigns with this player, including my friend's campaign, will be sabotaged by this behavior.
What can my friend do as a DM to help accommodate this player, and is there anything I could do as the problem player's friend to help them curb this problematic behavior?