Can you breast feed and formula feed?

Question

My sister had a baby 2.5 weeks ago and says that she is absolutely exhausted (understandably) she says she gets spurts of about 1/2 hour of sleep throughout the night and day. She was wondering if she would be able to formula feed the baby once or twice a day so that her husband could get up and take care of the baby while she got a few hours of sleep.

Can you do this or is it a really bad idea?