He's a cockerel, not a rooster. If your friend's hens are older, he will be low in the pecking order until he matures enough to start acting more mature. I've had one do that at 5 months, one didn't until 11 months. Most of mine manage around 7 months. Maturity makes a big difference in how they behave and onteract.
Will he be bullied? Maybe. It depends on the personality of him and the hens, how much room your friends have, and what integration techniques they use. Sometimes this is seamless, really smooth. Sometimes it gets violent. This kind of stuff doesn't come with guarantees. Once you make the trade, why is this your problem?
With the numbers in your flock I would consider this an excellent trade. I think you are staring at some really rough times in your flock over the next few months if you don't do something. Some people have two boys and three girls and it works but the odds are strong against you.
You will be bringing a single chicken into your flock of 3-month-olds. You have your own integration to worry about. That's what you should be thinking about. How old is she, how much room do you have, what do your facilities look like? Sometimes this goes really smoothly, sometimes this gets violent. If we know what you have to work with we may be able to help you with that.