Who has advised her to do this? I personally have been advised to up my protein to aid in muscle repair after running and strength training, so usually have one on a day I exercise. I also find red days up my loss faster than green, but that is just me as others do really well on green.
Most protein shakes I have are quite high in syns, I have some small ones that are 5 syns, and have just gotten promax diet which I believe is around 4 per shake.
If your sister isn't a veggie, she may just want to up protein by having more chicken, lean red meat, and fish, as they are all free on red and EE, or eggs on any day. It less expensive than protein shakes and syn free.
Hope that helps xx
Steph