wellbalanced Flucloxacillin and breastfeeding is ok. Commonly given for Mastitis and wound infections post-natally. cheesesarnie Difficult to comment withought knowing the particular antibiotic. Are you able to find out? Your friend hopefully has an enlightened GP who actually checks what can and can't be given when BF. The Breastfeeding Network are a good source of accurate advice and information, as is "Medications and Mothers Milk" edited by Thomas W. Hale 2008.
I am looking in a breastfeeding network booklet with info on drugs. Metronidazole says 'generally considered safe in breastfeeding, said to cause milk to taste unpleasant'. Could the other one be Azithromycin? Hale says it's fine: http://neonatal.ttuhsc.edu/discus/messages/40/423.html?1238090649
In both cases baby may get some loose stools and risk of thrush increases for mum and baby so she may want to get hold of some probiotics like lacto acidophilus capsules from the fridge in a health food shop.
Erthromycin is safe. There is one study that suggests it slightly increases the risk of pyloric stenosis (projectile vomiting is one symptom). The study looked at newborns. Breastfeeding Network has no hesitation in saying it's safe.