Disciple of Life only works when a spell heals as part of its description, not when something outside the spell does the healing.
The Disciple of Life feature states:
[...] Whenever you use a spell of 1st level or higher to restore hit points to a creature, the creature regains additional hit points equal to 2 + the spell’s level.
The Grim Harvest feature states:
[...] Once per turn when you kill one or more creatures with a spell of 1st level or higher, you regain hit points equal to twice the spell's level, or three times its level if the spell belongs to the School of Necromancy [...]
Disciple of Life applies when you use a spell to restore hit points to a creature and Grim Harvest applies when you kill a creature with a spell. The question then is whether the healing from Grim Harvest (technically a result of casting the spell) activates Disciple of Life; whether this counts as casting a spell "to restore hit points".
I would argue that no, it does not count.
This is because Disciple of Life requires you to cast a spell to restore hit points; not that you cast a spell, and that it just happens to result in the restoring of hit points; the spell must specifically be cast with the intention of restoring hit points. This is only the case if the spell's description states that it restores hit points, if the caster knows when casting the spell that it will restore hit points.
If you had Disciple of Life work with Grim Harvest, it would also work when a spell pushes a creature into a healing aura/area
If you allowed Disciple of Life to trigger from Grim Harvest, where the spell is not directly causing the healing (it has to go through the Grim Harvest feature), you would also have to say that a spell that pushes a creature into a healing aura/area activates Disciple of Life. Here a spell is cast and it ultimately causes a creature to gain hit points by going through another feature (the healing aura/area), it would not be directly causing the healing.
I believe this is a more clear scenario that the answer should be (and is) that Disciple of Life does not trigger, and thus, it similarly does not trigger with Grim Harvest.