If her OB said to stop there are LOTS of valid reason for that
- Bone loss (hers)
- Organ damage (hers or the fetus's -to differentiate from her 5mo old)
- Miscarriage
- Failure to thrive (baby's)
- and a few other reeeeeeally serious things
Now... these things are NOT true for everyone. In fact MOST women can continue to nurse while pregnant with no problems whatsoever. It's just fine. ***BUT*** for women with underlying health issues it's NOT fine.
Most women can exercise while pregnant. Some moms are put on bedrest. They CANNOT exercise or risk losing the baby/dying themselves. Just because most can, doesn't mean she should.
If she was planning on nursing for a year this is probably REALLY DISAPPOINTING news. Since hormones are in play she may end up being a wreck over it, or decide to ignore medical advice,
Please support and comfort her during this really disappointing / hard time. She'll have enough people saying "But my sister could nurse while she was pregnant!". She needs someone who understands her Doctor, the one with her blood studies and medical records and pediatrician records for her son has told her she needs to stop.