What happens if he is arrested for suspicion?

More information is needed. How did he did he become known to the immigration and customs enforcement Agency? Was he arrested for a crime? Did he enter on the visa waiver program, which means that he did not need a visa to come into the United States? Was he detained after the government determined that he was in overstay? In general, adjustment of status is a discretionary form of relief. That means that the government can deny an adjustment even if someone is admissible. However, the entire situation needs to be reviewed by a competent and experienced immigration attorney. There is a possibility that he may still be able to qualify for adjustment, but it may take more time. If you don't seek assistance soon, the situation may become more complicated. Good luck.