I realize this passenger is likely already traveling, but there is a huge risk in asking for a better class of seating due to a medical condition.
You may be asked if you are fit to fly on the day of travel. I'm not kidding.
When my leg was badly injured and I had to keep traveling to keep my job, I quickly learned that expressing the need for improved seating can get you a fast refund and the trip is over. I learned to respond that I was, indeed, fit to fiy and intended to do so.
This situation can be a tight rope walk for sure.