There's 2 things I've seen with Ardent Defender regarding Chaos Bolt. Firstly, the sub 35% damage reductions provided by AD do not mitigate CB damage, I'm not sure if this is by design or just an oversight.
Ardent Defender
Damage that takes you below 35% health is reduced by 30%. In addition, attacks which would otherwise kill you cause you to be healed by up to 30% of your maximum health (amount healed based on defense). This healing effect cannot occur more often than once every 2 min.
By the wording on the spell it's not an absorb but a static reduction. However it shows in the combat log as an absorb thus Chaos Bolt pierces right through it.
Secondly, and
I tested this a few times last night with the help of some cross faction friends. Once in a while if you are low on health and have a secondary absorb on you (Sacred Shield, PW:S, etc.) and that absorb is active, the healing effect from AD will not happen it will kill you.
With Chaos Bolt alone AD would always proc the heal, but rarely when you have some other effect on you it does not. I played with this for two hours last night trying to see if I could find the reason but it was quite random.
The majority of the time the heal worked as intended.