What are the implications of filing I-539?

My grandmother has been in the US for the last 5 months on a B2 visa. Her I-94 expires on 29th July. She wants to stay a little longer to attend a marriage function in October. She plans to leave after that. We are planning to file I-539 to extend her stay with a return ticket attached.

If I-539 is denied, will she accumulate an out-of-status stay since the original I-94 expiration date? Will that be a problem if she plans to apply for an immigrant visa?