Warding Flare and 5.5 Rules

Use of Warding Flare Questions: 

Rule says, "When a creature that you can see within 30 feet of yourself makes an attack roll, you can take a Reaction to impose Disadvantage on the attack roll, causing light to flare before it hits or misses." 

1. Can I use it to impose disadvantage on a creature attacking an ally and not me, as long as the creature is in range? 

2. When do I have to declare I'm using it, before the DM makes the roll or before the DM announces the result? 

1. Yes as long as they’re in range. Though it is “a creature you can see”, so if they’re invisible or behind total cover or something, then you can’t use it.

2. Before the roll. It imposes disadvantage, which means it’s a part of the roll. Unlike something like silvery barbs which forces a re-roll.

Quote from Xalthu >>

2. Before the roll. It imposes disadvantage, which means it’s a part of the roll. Unlike something like silvery barbs which forces a re-roll.

It should also be noted that it explicitly says "before it hits or misses" in the text of the feature. The whole point of that phrase is to make it clear that you do this before you know the result of the attack.

Also note that #1 is a change that was made for the 5.5e version. If anyone is telling you that you can't do that, they are probably thinking of the 5e version, which only worked on attacks made against you.