Without knowing the extent to which the person's left thumb is disabled, it's impossible to know whether her left hand can hold down the strings tightly enough to play clearly. If that's a real problem, playing opposite-handed ("lefty") is the only real option, and would certainly trump the more subtle consideration that one's primary hand should play the relatively simple role of strummer/picker in order to "lead" the other hand through the rather demanding process of fretting..

Thanks to Kevin McGrath for informing us that Django learned to play the guitar before losing those fingers. I never knew that, all I had known was that he famously played without an intact left hand. That insight certainly explains a lot; I think that it's very unlikely that a person could become such a notable artist on the instrument had they suffered from such a limitation even when first learning to play.