When you first meet him, he will advise to go talk to Captain Mercaptain. After you talk to her the first time, he will clal you over and offer to sell you some guns. They however are defective and have a very high chance of jamming unlike what he claims.
If you refuse to buy any of his guns, he'll tell you he needs the money for his sister Katy if you question him about him needing money. If you question him further, he'll ask you to take a letter to her in Rail Nomads Camp.
You can convince Robert to leave the Desert Rangers and go be with her sister after talking about his letter. If you tell Mercaptain about his actions, she'll have him arrested by Ranger Gunners.