Dear Brenda,
With your mom's history of binging and purging, there may be her own dietic
reasons for the low bone density.  With anerexia/ bolemia women can go
through long periods of amenorhia which can affect the bones.  Maybe some
counselling with a dietcian to help increase her calcium.  I would strongly
encourage her to breastfeed these expected baby and give all the reasons why.
Good luck.
Ann Perry