Can a frequent business visitor to the U.S. get a U.S. airline credit card without an SSN?

Overview

My dad frequently travels to the U.S. on business and has a valid B1/B2 visa. These days he's visiting almost once every 3 months, and when he's here he also flies domestically quite a bit. It got me wondering whether it would make sense for him to have an airline credit card, such as one from United, American, or Frontier, to earn miles and get perks. The catch is that he doesn't have an SSN and isn't a U.S. resident. I live in the U.S. and have several U.S. credit cards, so I w

SpinGraph analysis pending — check back after processing.