Is he not adjusting his status from K-1 by filing I-130/I-485 and I-765? How's this any different from someone filing AOS from a different non immigrant visa?
Why do you say it's a disadvantage of K-1 when they filed for an AOS? You mean like more scrutiny or something given that they don't have a longer presence in the US?
Just curious, was he out of legal status but was on an authorized stay because his mom filed a petition for green card? and do you know approximately how long after the visa expiry the green card petition was filed?