With a feather from the right wing instead of the left.

It would certainly have been a struggle, as you say, which is why it was also pretty rare among professional scribes. Left-handed writers were forced to push their quill, instead of the more natural pulling motion, and they would indeed have issues with rhythm and ink smearing, making the whole process much more slow and laborious. That being said, it was certainly not completely unheard of during the Middle Ages, and wasn't even necessarily shunned. For some, writing left-handed would have granted additional pride in their work, especially significant considering the religious nature of writing at the time. But those bragging rights wouldn't have been earned easily.

By the way, I just came across the story of Raffaello da Montelupa, a 16th century Italian artist who apparently wrote from right to left to cope with his left-handedness, and amazed on-lookers with the legibility of his script. Maybe tell your friend to try that if the tradition left-to-right proves too taxing.