You can do it how its meant to work, having the user choose the effect by using a query.
If I understand correctly, one player has uses the ability, and another player then pics from two options?
You can do this with two Abilities.
Call the first one, Glimpse-of-reflection-Trigger, and the macro looks like this:
&{template:default} {{name=Glimpse of Redemption}} {{Trigger=An enemy damages your ally, and both are within 15 feet of you.}}
{{Effect=[@{Target|character_name} Click Here](~FRODO|Glimpse-of-redemption-Effect)}}
The linebreak is for clarity - remove that in your ability.
Change the word FRODO to the name for the character who has this ability.
Now create a second ability on the same character, called Glimpse-of-redemption-Effect
&{template:default} {{name=Glimpse of Redemption Effect}} {{Choice=?{Pick One|Avoid Damage,The ally is unharmed by the triggering damage|Gain Resistance,The ally gains resistance against the triggering damage equal to [[2 + @{level}]]}.}}
The way it works is. The character with the ability triggers it, and they are prompted to select a target (who must have a token). Imagine they choose a character called Faraam. This is printed into chat:

That is a button which can be clicked. When Faraam clicks it, they rae given the option of choosing to avoid harm, or gain resistance. If they choose gain resistance, it'll print out this:

I've assumed both macros go on the Abilities section of the character who has this spell. If they are in the Macros tab, you need to change this part in the first macro:
[[2 + @{level}]]
to
[[2 + @{FRODO|level}]]
Again, change the FRODO part to the character name.
If you want anyone to be able to use this effects, change all FRODO references to selected.