So do you think it will be safe for her? I have been told i have to add two or they might be diseased.
That's not how it works. But there are two different concerns.
One is integration. May of us integrate new chickens a lot without too many issues but it can go horribly wrong. The more room you have usually the easier it is. Room is very important but even with plenty of room in the coop, on the roosts, and outside you don't get guarantees. Maturity is another big factor. Maturity is kind of related to age but isn't exactly the same. Typically when pullets start laying they can be considered to act maturely. Since you want a pullet or hen already laying so you know egg shell color I won't talk about maturity differences that much, that's more complicated. If you decide to get immature chicks we can talk about that later.
It is sometimes beneficial to integrate more than one new one at a time. That's not as important when they are all mature as it is when the new ones are immature. Chickens are social animals and like to have buddies. When you integrate mature chickens they tend to settle the new pecking order pretty quickly. It can get violent for a short time but it's usually over pretty quickly. But sometimes the new ones like to hang out apart from the others, especially at first. If you have enough room it just often works out better if you add two instead of one. That may be where the "two" came from in what you heard. This has nothing to do with disease.
Any time you meet a new person, either one of you may give the other a disease even if you don't shake hands. It works the same way with chickens if one is diseased. You don't have to meet two new people, one is enough if one is diseased. The great unknown is if any of the chickens is diseased, yours or the other. Parasites can also be a concern.
It is not that unusual for chickens to develop what we call flock immunity to either a disease or a parasite. They can be infected yet never show any symptoms so they can be carriers. It could be the flock the chicken is coming from or it could be your flock.
Or it is possible that a chicken has recently just been infected, especially if it recently met a new chicken. Chickens in what we call a closed flock, where they are kept away from other chickens, are not very susceptible to this, but they may still have a flock immunity. Chickens you get at auctions or chicken swaps are at a higher risk.
Some people just bring in new chickens without worrying about this. Often it works out fine but it is possible you could bring in something that wipes out your flock. When new birds do bring in something (which is not that often) it is usually something like mites or lice. You can treat that so it is more of an inconvenience than a disaster. But it can be a disaster.
Some people quarantine new birds, keep them isolated for a month to see if they come down sick. It's also a good time to treat them for mites or lice or even worms even if they don't show symptoms. But a good quarantine is not easy to do. Diseases and parasites can spread by them eating or drinking from the same containers, pecking at the soil where they have pooped, by insects, or on the wind. You can transmit it yourself if you wear the same clothing, especially shoes, when you are feeding or watering them. The better your isolation the better the quarantine. Very few of us have the facilities to handle a good quarantine.
I strongly recommend a quarantine for any birds that come from an auction, show, or a swap. These birds have been exposed to strange birds. If the bird comes from a flock that has not been exposed to new birds the risk is much less, especially if the owner would recognize a disease or parasite and was honest enough to tell you about it. That does not mean there is no risk, just that it is much less.
Would your girlfriend be willing to swap?
