Simply stated, you cannot. If you failed to turn in a hard-copy I-94 upon leaving the US, there are procedures to mail it in, along with corroborating information, to get your records updated. However, if you did properly turn it in (assuming you had a hard-copy, they are automated these days, when entering via airlines), there are no provisions for verifying that your records were properly updated.
If you and your father are truly worried, take what documentation you may have showing he departed the country on time. That is, unfortunately, all you can do at this time.