I made a shirt for a large relative. Mine was also a gift. I make a copy and changed it to a yoke style and added a generous pleat in the back. I felt the pleat gave enough ease and still looked neat.

http://www.sassyscrubs.com/store.asp?pid=5163&catid=19621
http://www.mccallpattern.com/charts/bwomen.htm

I wear size 22W which is usually 2X. According to a catalog size chart I have, size 3X is 24 to 26W. You may want to go with the 26 as scrubs are usually of woven fabrics and nurses need room to move!
Eunice

A 3X is a 26/28 size
Izzysmom

Thank you so much for everyone's response. I was absolutely stumped! I knew I could count on some sewing buddies to know!

One of the problems I run into being plus sized is what I wear in RTW is smaller than what I wear in patterns. I.E. I can wear a 20-24 in RTW but have to have up to a 28 in a pattern because of my hips. If she wears a 3 x in RTW, take your measuring tape and measure a jacket that size and then look at those measurements on the pattern jacket and see if that helps you determine the pattern size.
Susan in Bama

If possible try to measure one of her jackets when she's not around. You know how much difference there is between manufactures. I wear a smaller size at high price stores than at low price. I've noticed the 'vanity' sizes in RTW. There is no way that my hips have gotten smaller over the years but I'm still wearing the same size jeans. I've got a retractable tape I carry with me.