Mary Alice:  I have a few references about rheumatoid arthritis.  The
first two showed an association between flare-ups or new onset of RA with
breastfeeding, but the third one shows decreased mortality and morbidity
from RA in mothers who have breastfed:

Brennan P, Silman A.  Breast-Feeding and the onset of rheumatoid
arthritis.  Arthritis & Rheumatism 1994;6:808-13.

Jorgensen C, Picot MC et al.  Oral contraception, parity, breast feeding,
and severity of rheumatoid arthritis.  Annals of the Rheumatic Diseases
1996;55:94-8.

Brun JG, et al.  Breast feeding, other reproductive factors and rheumatoid
arthritis.  A prospective study.  Br J Rheum 1995;34:542-6.

These studies were with small numbers, and the postpartum period is a
common time for onset of RA, so it's hard to draw any firm conclusions.
However, IMHO, it's fair to say that it's possible that she may have a
flare-up while breastfeeding but long-term may have less morbidity.
If she should have a flare-up, there
are medications which are compatible with breastfeeding (I had one patient
who had her first episode of RA at 1 month postpartum, and did well on
prednisone for several months; although she could also have used plaquenil
[hydroxychloroquine], she didn't want to because there was a genetic
retinal condition in her family; she went on the plaquenil after weaning
the child, and has done very well).
Hope this helps.  Alicia Dermer, MD, IBCLC.