A player in my game came in with a silly name for his half-orc, until I got the backstory. Growing up destitute in the city he would taunt the baker's children into calling him names and eventually chasing him off with thrown baked goods just so he had something to eat. One of the often used names was shortened and he was known now as Biscuit. Best silly name made good.
There's a player in our group who is low-Cha IRL. I mean, he's a nice guy who means well, but if there's an absolute worst way to put something, that's the way he's going to say it. When he's playing a high-Cha PC, I encourage him to just roll the Diplomacy dice and tell me what he wants to get out of it. Anything he actually says when he tries to roleplay it out just makes me want to give him penalties.
You just have to work to flip that around. I'm sure you can imagine ways your friend would say things IRL compared to things other, more charismatic, people say. You just have to take the horrible way the player puts it while roleplaying and apply the result of his Diplomacy check to "translate" it into what his more charismatic character would say. No need for penalties (or bonuses IMO), just relate it to what the dcie tell you. You may get the flipside where a well-spoken player may roll a 1 with his CHA 6 barbarian. Then you just think about how your other friend would say it that would offend the people he's trying to speak to.