Semen ???

I am not personally sexually active but my best friend is and I'm trying to get some answers for her. I always read on here that if your guy has recently cummed before y'all get busy then there can be remnants of semen in his pre cum. How long before hand would he have to cum for it to still be there and how long should they wait to have sex to reduce that risk. As much as I preach condom usage to her, she prefers to go without so any advice on this subject is appreciated. Thanks in advance babes!!! ❤️