I fear that you might be attempting the impossible but I'd try a drop of white spirit on the inside of the cover first (to check that it's not going to completely ruin the surface of the plastic) and then try to remove the ink with it.
If all else fails mabye buy a cover to cover it hundreds on ebay...i recommend a silicone one if hes a bit careless ...cos it keeps the face of it off the floor when dropped lol xx
It's from a broken ballpoint (which he has only just told me about - apparently he had a load of duff ones that leaked). Might buy him a new one to cheer him up (it was only a couple of quid off eBay.) Thanks for your answers.
Just to clarify - I will get him a new cover (as he is so disappointed that it is marked) and I will him buy some new pens, the iPhone was his dad's old one and the cover to go on it was only a couple of quid off EBay (which he paid for himself).