Communicate to your friend with the new baby that you're there for them, regardless of what they need or when they need it. Check in regularly, and ask that they promise to be open with you if there's anything that can be done to make this transitional phase easier. You don't have to hover, but a simple text or phone call now and again can go a long way.
Just knowing that someone else "gets it" and understands that parenthood can include moments of unparalleled bliss and those of sheer distress (sometimes in the same day!) will be incredibly comforting.
