The tether essence spell states:
Two creatures […] are magically linked for the duration, regardless of the distance between them. When damage is dealt to one of them, the same damage is dealt to the other one. If hit points are restored to one of them, the same number of hit points are restored to the other one. […]
Meanwhile several healing spells cannot affect Undead, for example, healing word:
[…] This spell has no effect on undead or constructs. […]
But what happens when an Undead and a non-Undead are linked together and healing word is used on the non-Undead individual. Obviously, they would heal, but what about the linked Undead; do they heal as well?