My crazy best friend

No Judgement please!
So my homegirl is a bit concerned.Since she has been single her and her ex have still been fucking around every now and then.She has also been messing around with another dude who she's known forever too.But the other guy has never came in her versus her ex bf does every time they have sex.
She's concerned that she may be pregnant due to her cycle being a no show as of today.
Now if she is and keeps up with dates frm both guys how can she determine who will b the father??????!?
Again no judgement or negativity plz