You don't need to appeal to a Druid's alignment for an answer to this. You will find the general answer on page 65 of the PHB in the Druid class description.
Druids accept that which is cruel in nature, and they hate that which
is unnatural, including abberations(such as beholders and mind
flayers) and undead (such as zombies and vampires)
Does that need to create a conflict between the Druid and another player, or another player character? It depends. The Druid could be a bit of a rogue Druid, or operating under a curse, or something else and thus mitigate the usual opposition to undead. Or, the players can develop that "common good" theme so that the goals of the Druid align with the Necromancer. Something to sort out with the DM at your table, and with the other players. As the campaign goes on, this can create dramatic tension. It depends on how each player plays the characters' roles.
We had a wide ranging discussion on a GITP thread regarding Orcus, undead, and if a Druid would help out someone leading undead.
My take was no, due to the statement in the book.
The folks at the table had fun.
In 5e, working things like this out at the table will prevent friction during play. There's a lot of room to work.