If his head is stuck as a pussy for 1 week, he won't have a body to return to. His body that's laying on his bed at his house, has a pussy, instead of a brain. His body will die of dehydration in 3 days. So at the end of 1 week, does his head swap to the corpse that was his body? Or, more likely, he's permanently stuck as that girl's pussy.
Yes, he gets to experience extreme pleasure. However, the tradeoffs are awful. He's going to spend the majority of each day in darkness, inside her panties. He has zero control over his life. He can't eat any food, other than semen. He has to piss out of his mouth multiple times each day. During the girl's period, he'll be bleeding out of his mouth. And what if the girl decides to date a dude? His life is going to suck badly for the vast majority of his life; with only fleeting moments of pleasure to look forward to.