Not read all thread(yet), but OP didn't say that her friend suggested leaving her baby 'overnight', she said she could go out for the evening as the baby slept through the night.
It is wrong though and is something that needs to be addressed one way or another. I've been a single parent since I gave birth 10 years ago and even now, I wouldn't leave my Dc's alone for a couple of hours, leave alone a young baby for an evening.