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Heartache by the Number - Recruiting Cassidy
You can recruit Cassidy through a bit of a related quest. You can start it out by talking to Cass in the Mojave Outpost Barracks. She's sitting at the bar. She can't offer any caravan work since her caravan was destroyed. You'll probably run into it on the way up to the Vegas Strip. She'll point you to the Crimson Caravan office if you want some caravan work.
This office is in the Crimson Caravan compound near the Strip. Go inside and talk to Alice McLafferty. You'll need to do a quick mission for her and deliver an invoice to Dr. Hildern. After you do this, talk to her to get a new set of missions. One of these will involved talking Cass into selling her caravan (which is just burned ash). With a Speech or Barter check, she'll be willing to sell. Once the title has changed hands, she will be recruitable as a companion.
Cassidy actually has the easiest companion quest, since you don't have to do anything extra for her. She'll just ask you to help her out as soon as you recruit her.
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Heartache by the Number - Investigation
Cassidy will want to investigate the ruins of her caravan to pay respect for her men. You probably already passed it on the road, so just travel to it or a nearby waypoint and walk up to the ruins. She should start a conversation noting that she hadn't connected "burned to ash" with energy weapons. It doesn't look like a bandit attack, so we have a little more work to do.
There's another caravan that was hit in the northwest. You can go around the strip to get to it. Watch out for nearby cazadors though. These guys pack a big punch at any level. Once you make it to the Griffen Wares caravan wreckage, just approach the pack brahmin to trigger another conversation. Cass will point out that they were attacked by energy weapons too.
There's just one more caravan to check out. It's a far ways off to the east of the Strip. Walk over to the marked location across the bridge to find Durable Dann's Wrecked Caravan. This one is a little more helpful. There are two dead Van Graff thugs and a Crimson Caravan thug. Go ahead and loot their bodies while Cass examines it. She'll tell you that this officially seals the case. She wants revenge.
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Heartache by the Number - Revenge
Cass wants to just kill both of the leaders. Alice McLafferty is easy. If you've got a silenced weapon, you can probably just sneak in at night and shoot her in the head a few times without anyone knowing. Gloria Van Graff is a little harder. You should probably work with her for a bit under "Birds of a Feather." If you do the first quest, she'll trust you as a guard and you'll be allowed to bring weapons inside. You'll probably just have to shoot her. Another option is to take up the job of a security guard, but let the last man into the store (the one that "declares" his revolver). He will suicide bomb the store and take care of the problem for you. Otherwise, you can advance Birds of a Feather until Jean-Baptise asks for you to bring Cass into the store. He wants to kill her, so that will trigger a gunfight. The benefit is that you'll be somewhat hidden away in a side hallway.
Killing Alice and Gloria will cause Cass to pick up the "Heart of Vengeance" perk. She'll do more damage with guns.
If you want to do things the lawful way, you can gain evidence of their conspiracy. The Crimson Caravan's part is easy to find. The safe is fairly hidden and only requires 50 points in Lockpick. You can probably just crouch by her desk and pick the lock. You can also pickpocket the key off of Alice. Take the holotape and show it to Cass to confirm the first part of your evidence.
The Van Graff evidence is a little harder to get. You'll need 75 points in Lockpick or an office key from one of the guards Gloria. You will also need to use a stealth boy, since there appears to be a guard by the office door at all times. Use the stealth boy to get inside the office and then pick the lock on the safe. If you are able to steal the key and don't have the points in lockpick, it's possible to open the safe by hacking the nearby computer with 75 points in Science. You can get the letter from the safe to confirm the conspiracy.
Cass will suggest taking it to Ranger Jackson to make sure it reaches the right hands.
Delivering the evidence to Ranger Jackson at Mojave Outpost will convince Cass that the bureaucratic nightmare she just unleashed is far worse than merely killing a few of their leaders.
Giving the NCR evidence of the trader's crimes will give Cass the "Calm Heart" perk, which gives her another 50 HP.