Where did she get the egg?
Hooded Mergansers are one of the most difficult species of ducks to raise in captivity, most zoos don't even keep them because they're so tough. They're also protected by the Migratory Bird Treaty Act, so if it's from the wild it's illegal to be in possession of.
For your original question though, absolutely it will need a friend if it has any hope of survival. It won't start eating unless there's another one with it to show it how.
And the friend can't be a domestic species, unless it's maybe a Call or something. Any other domestic duck would grow too quickly, and could end up hurting the duckling. I would look for another wild species of duck for it.