The warlock's Armor of Shadows eldritch invocation (PHB, p. 110) says:
You can cast mage armor on yourself at will, without expending a spell slot or material components.
The School of Abjuration wizard's 2nd-level Arcane Ward class feature (PHB, p. 115) says:
When you cast an abjuration spell of 1st level or higher, you can simultaneously use a strand of the spell’s magic to create a magical ward on yourself that lasts until you finish a long rest. The ward has a hit point maximum equal to twice your wizard level + your Intelligence modifier. Whenever you take damage, the ward takes the damage instead. If this damage reduces the ward to 0 hit points, you take any remaining damage.
While the ward has 0 hit points, it can’t absorb damage, but its magic remains. Whenever you cast an abjuration spell of 1st level or higher, the ward regains a number of hit points equal to twice the level of the spell.
Mage armor is an abjuration spell.
Can a warlock/abjuration wizard with the Armor of Shadows invocation proc Arcane Ward at will?