One of my players is playing a Ranger (Monk 7/Ranger 2) who has Orcs as her favored enemy. I plan on throwing an Orc Revenant at her and the party. Would her favored enemy ability activate when fighting an Orc Revenant (or any Undead Orc)?
No, it wouldn't
Favored Enemy states:
Choose a type of favored enemy: aberrations, beasts, celestials, constructs, dragons, elementals, fey, fiends, giants, monstrosities, oozes, plants, or undead. Alternatively, you can select two races of humanoid (such as gnolls and orcs) as favored enemies.
An orc is a humanoid. Once it dies and comes back, the orc becomes something else entirely: it becomes an undead. Your ability to best track orcs is useless if it becomes a skeleton or a floating ghost.
If you do allow this, expect players to soon ask why they can't do the same thing in reverse, e.g. "My favorite enemy is undead, a zombie is just an undead human, so why don't I get my bonus against living humans?"
By RAW, The Revenant is undead not humanoid. Thus, your Revenant Orc would be a medium undead not a medium humanoid (Orc).
A monster's type speaks to its fundamental nature. Certain spells, magic items, class features, and other effects in the game interact in special ways with creatures of a particular type. For example, an arrow of dragon slaying deals extra damage not only to dragons but also other creatures of the dragon type, such as dragon turtles and wyverns. (MM p7)
However, you as the DM have the last say. If you want to say that the Revenant Orc is an Undead Orc, you are within you bounds to allow Favored Enemy to be triggered by it.
As mentioned above: No. However, are we just talking about the following abilities: "You have advantage on Wisdom (Survival) checks to track your favored enemies, as well as on Intelligence checks to recall information about them."?
The rules say "The GM can also decide that circumstances influence a roll in one direction or the other and grant advantage or impose disadvantage as a result", so I might see a DM giving out advantage to track an undead orc that was still behaving very orc-y (which a revenant might).
The favoured enemy works off the monster's type as indicated in the stat block.
So the answer depends on whether turning the monster into a revenant changes the creature's type or not.
If you are applying the revenant subrace from Unearthed Arcana (the only source I can access!) then there is nothing in there that states the creature's type changes. So the answer would be yes, the favoured enemy bonuses would still apply.
If you are applying some other kind of revenant that I don't know about, but which specifically states that the creatures type changes to Undead, then the answer would be no.
NO, unless you are playing the original Ranger and have chosen Undead as a new Favored Enemy at either 6th or 14th level of the Ranger class, then the rules for that feature would apply.
Current rules on Favored Enemy in the Revised Ranger are as follows:
"Choose a type of favored enemy: beasts, fey, humanoids, monstrosities, or undead."
Greater Favored Enemy (6th level) of the Revised Ranger states:
At 6th level, you are ready to hunt even deadlier game. Choose a type of greater favored enemy: aberrations, celestials, constructs, dragons, elementals, fiends, or giants.
The Revised Ranger does not allow the option of Undead at 6th level, and has eliminated the additional enemy at 14th level.
So, your ranger has Humanoids as her Favored Enemy and although the Revenant has the form of an Orc, it is now an undead, acts like undead, has the undead vulnerabilities, not the qualities of an Orc. For example, hitting an Orc in the balls is a critical, just like the other Humanoids, but since this is now an Undead, this is no longer a critical, so the knowledge of Orcs becomes irrelevant, or even dangerous. If she were to badly fail a knowledge check, then you could tell her that crits to Revenant Orcs still work normally...