Hello Mr. 8,
I just checked her W-94 and the expiration date is 07/04/2016; I wonder whether it is very risky if I put her fly ticket on 07/04/2016. You know, it is possible that her fly can be delayed.
-She had better leave in advance a few days before the I-94 expires, just in case.
I mean if that is the case, can I use that excuse to argue with USCIS for the validity of her B1/B2 Visa?
-No. You can not argue with the law. USCIS does not control the visa validity, but the CBP officer at the port of entry does, and your mother will not be allowed to enter US if her visa is not valid (CBP will record the date of your mother's departure from the Airlines).
Thank you very much!