I'm going to disagree with oldhens, on this one. I have my Ducks and geese together in one area, and in the coop I have Chickens, Guineas, and Turkeys. Yes, the Guineas can be pretty aggressive especially this time of year, but the adults seem to handle it fine. I also free range during the day and have a rather large run attached to my coop which may help. I'm not saying that the OP is doing anything wrong at all, in fact quite the opposite. I have been doing this for years with guineas and chickens living together. It's totally doable. However you have to do something or your pullet is going to get seriously injured or possibly killed.
You don't really say the age of the hen other than "pullet" so I know it's young but is it full sized? pretty close? still very small? That info would be helpful. Your going to have to remove one of the two birds right away, but that doesn't mean you can't reintroduce them later. Remember right now is laying season, setting season and mating season. If the guinea is old enough to mate, then that is part of the problem, especially if the guinea is male and doesn't have a mate. In my experiences Guineas usually pair off one male, and one female and they seem to stay as a mating pair for life for the most part. I have no scientific evidence of that it is merely my observation through the years.
If it were me I would put the chicken that is being picked on into you brood pen. Sure she'd be alone, but she wouldn't be getting picked on anymore, she would would be able heal any injuries and she'd have her own food, water and fresh grass. This will enable you to see how the Guinea behaves with the rest of the birds. If ok then that's great. I'd probably try moving the hen back in, in a few weeks or a month. If the Guinea finds another bird to beat up on then I'd pull the Guinea, and move the hen back in. Again I don't know the age of the Guinea other than young, so I'd assume this is the first mating season, and again I'm assuming he's actually a male. Give him a month to mature and then reintroduce him. One of these methods will more than likely work. Having an even number of male and female Guineas is a HUGE plus, and having more females than males is fine, but never have more male and female Guineas that's going to be an issue.
Another thing you mention that they were raised together, so again going down the thought that the guinea is male and without a mate, he may see the hen as being a funny looking guinea and might not understand why she won't mate with him.
If you don't know how to tell male and female Guineas apart let me know, you can tell just by listening to their call and I can help you identify.
