Normally how many days a sperm can live in female body.

Because my frnd has intercourse on August 5 after that next period date on August 15 but she didn't know can she get period or not. After that she had again intercourse on September 8 onwards continue many days.

Some body told that August 5 intercourse has chance for pregnancy but its sperm is stay remain constant and after sep 8 intercourse it becomes harder.is it possible