If I were close friend's with someone else, I don't think I'd mind. Sure I'd prefer it if I gave permission first but I don't think I'd be furious.

Interesting side note: back in that 80's before mobiles, my mum was baby sitting a close friend's breastfed infant who was the same age as my then breast fed brother. The mother had fed the baby before she left but the baby just would not settle at all for my mum and was crying for ages. Walking, rocking, nothing would stop the baby crying. So as a last resort, Mum tried to breast feed him. Apparently he attached for a while and then pulled off, refusing to feed . Like he knew that it wasn't his own mummy's milk or breast. So I wonder if a baby older than a few months would even breast feed happily at another's breast.