This medication should not be of too much concern.  I am not sure why
Hale says use with caution.  If there is a real worry, Asacol, is safe.

What needs to be taken into consideration is that there is evidence that
breastfeeding help reduce the risk of subsequent inflammatory bowel
disease in the child.

There are other risks associated with not breastfeeding.  The question,
as was stated previously on this list, is:  Is breastfeeding with some
sulfasalazine less risky than not breastfeeding at all.  Sometimes
philosophy and a point of view about life colours the answer.  In my
book, there is no doubt that the mother is better to continue
breastfeeding.  However, she should get as much information as she can,
as I presume she is doing, and then, the final decision is up to her.
There are no clear cut answers except very rarely.

Jack Newman, MD, FRCPC