Reporting him is a caution. He may not be a hacker, but it's always safer than sorry. If he isn't a hacker, then the GMs will bring the issue up and address it in public based on my past experiences.
Bonus Feature:
I'm not sure on the damage calculations, but I do know that crits can't multiply or otherwise I'd be able to do that much damage too and even more pretty easily.
And if we take it into theoreticals:
Code: 
Assume crits multiply it by 2x. That means it's a total of 32x as a crit.
Lets go with 5 Rrarities boosted up to the maximum. This is about 20k total.
If it crits, it'll be 20k * 32 = 640k. From what I can tell, the resulting damage and crit are then multiplied by 5 and then divided by the amount of enemies in the opposing vanguard team.
This means that if there's 1 person, it's 3.2m damage, 1.6m, 1.1m, 800k, and 640k going down the line.**
**: With a reasonable variance in damage. Probably around 10%.

Code: 
Assume crits don't multiply and is additive. That means total of 10x.
Using the previous values, it gives us 1m which turns into 1m, 500k, 333k, 250k, and 200k.**
**: With a reasonable variance in damage. Probably around 10%.

Code: 
Assume there's only one crit calculated for damage. This means 2x.
With the previous values, this gives us a grand total of 200k which turns into 200k, 100k, 67k, 50k, and 40k.**
**: With a reasonable variance in damage. Probably around 10%.

In the OP, the person stated was level 25 and most likely didn't have uber buffed up guards because it would require a grand total of 45k damage. If crits did multiply, then he would need SRs or MRs leveled to the maximum for that effect.