急問：criminal record, visa and green card

I have a cousin, who is currently on H1-B visa in the US. He was involved in a charge on domestic violence (assult), which has been dismissed and expunged now.
Can any of the attorney kindly explain: how his stituation will affect his re-entry into the US and his future green card application? Also, how will it affect his parents application to visit the US?
Really appreciate your kind help!