Force him to tell the Underworld residents the truth.
Force him to only sell legitimate Aqua Pura from now on.
Griffon, a ghoul outside of Underworld, is selling Aqua Cura, a tonic that he claims will cure the ghoul condition; however, you learn that he buys Aqua Pura from Bigsley and replaces it with dirty water, re-branding it Aqua Cura in the process.
There are four ways to begin this quest. The first option is to talk to Griffon in the Museum of History and ask for a bottle of Aqua Cura. According to your Pip-Boy, though, the water is irradiated. If you bring this fact up to Griffon, he will deny the claim and the quest will be added. It can also be added by either passing a speech check with Scribe Bigsley or by hacking his terminal. The fourth option is to stumble upon Griffon's bottling operation located in the Museum Authority Building, find the holotape and listen to it. Using any of the above methods will add the quest.
Find Griffon's bottling operation
To find proof, you need to find Griffon's bottling operation. Naturally, the ghoul salesman himself does not want to tell you where it is. However, as he mentioned that Scribe Bigsley's men deliver the aqua pura to him, Bigsley likely knows where it is. Arriving at Jefferson Memorial, the scribe will freely tell you that Griffon's operation is in the Museum Authority Building. When you first enter the building, there is a bathroom on the right. There is a bottle of Nuka-Cola Quantum in the tank of the toilet in the closed stall. Follow the hallway on the left of where you entered the building, heading to the northwest corner of the ground floor and go down the stairs. When you go downstairs take a right into a big room with barrels and tubs. In between two tubs on a coffee table is a locker full of wigs and a holotape. An identical holotape of Griffon's bottling instructions can also be found on one of the ghoul guards in the basement. In the same room, you can find 4 crates of aqua pura, just to the left of where you found the holotape. The holotape tells you that Griffon's ghoul guards dump the Aqua Pura into the tubs and sinks, selling that to raiders and slavers while he sells dirty water in Aqua Cura bottles to ghouls.
Dealing with Griffon
When you go confront Griffon you have three options: blackmail him, force him to tell the truth, or force him to sell real Aqua Pura from that point on. None of the choices affect your Karma.
You can blackmail him for 300 caps single payment or 40 caps every (in game) week. Alternatively you can pass the first speech check for 100 caps every week, the second speech check for 80 or the third for 60. The PC need not visit Griffon every week to receive payment. He will still pay the PC for missed weeks (e.g. if the PC waits for 1 in game month, Griffon will pay 4x the one-week payment).
If you choose to blackmail Griffon and pass the first speech check for 50/50 profit shares with him (which has a low percentage rate for success), after this dialog with him ends you may immediately initiate another one whereby he'll give you 200 caps on the spot, mistakenly (and inexplicably) thinking he owes you back pay for having been away. Thereafter, you will have to wait for the weekly payments.
Forcing Griffon to confess
If you force Griffon to confess about the true nature of Aqua Cura, the crowd will turn hostile against him and:
Griffon will attempt to defend himself, but ultimately run away and hide in one of the bathrooms at the entrance of the Museum of History. The crowd will follow him into the bathroom where Griffon will make a final stand but is eventually killed by the angry mob. After which you can loot 600 caps off him.
PCXbox 360 He may be killed by the mob at other locations in the Entrance area. He may also escape the mob and flee through the doors to the Lower Halls. The mob will follow him, but they disappear after entering (apparently they can exist only in the Entrance). Griffon will be just inside the Lower Halls with gun drawn and will remain there as long as the player is nearby. He eventually disappears from the area if he is not killed by the player.
Selling the real aqua pura
Although Griffon will tell you to come back in a few days, his dialogue will remain the same, simply informing you his business with you is concluded, then try to pass you off as a happy customer to the crowd.
If you go to the Museum Authority Building and take one of his wigs from his closet and put it on, you get a special dialogue option in which you say that you and he have the same hair cut. Griffon will try to write it off as a coincidence.
You can also loot the 600 caps and not turn the crowd hostile toward you by mezzing him.
If you have sent Fawkes back to Underworld and choose an option that turns the crowd hostile, Fawkes will then go hostile with the crowd and help you out.
Shooting Griffon with a dart gun while hidden will cause Fawkes to murder everyone there, should he not currently be your active companion.
If the player kills Griffon, or he dies any other way, then a dialog option will be available with Scribe Bigsley to tell him that Griffon is dead.
Even if Griffon agrees to sell the actual aqua pura, the water will still be irradiated.