I really don't think it's a good idea. There is a chance that she may be pregnant / labouring / giving birth on that day. Less than 10% of babies are born on their 'due' date, which means that about 45% arrive 'early' and 45% arrive 'late'. Just because her first was early absolutely doesn't mean that her second will be.
Apart from that, as has already been mentioned, she really may not feel up to going out that soon after giving birth. She may well have felt fine five days after having her previous child, but every birth experience is so different there's no way of telling how she's going to feel this time round.
I'd say leave it and book something once the baby's arrived and she's feeling well enough to go out.