After your DIL married to your son, she could be on an international student visa and applied for PR status change the same time. It will take up to 12 month processing to get marriage green card. She could provide her status change information to medical schools and after verification, they could consider her as a PR and gave her an exception. I think that is what happened.
It is a fact that only 106 “true” international students matriculated in 2017-8 application cycle. It is on AAMC’s record, I am sure AAMC is not going to lie.
https://www.boundless.com/immigration-resources/how-long-does-marriage-green-card-take/