Implantation happens on average 6-12 days after ovulation. It can take less time and it can take longer. That is just the average. It takes 2-3 days for enough hcg to build up in your system to get a positive on a home pregnancy test. It could be early. It could be stress. If ovulation wasn't tracked it could be that ovulation happened later in the cycle than usual and that will make your period late also. I would wait a couple of days and test again. My doctor won't do a blood draw or see me until it's been 2 weeks without my period or a positive.

I say stress. I would test a week after missed.

It could be stress or still to early for them, I've had this happen for both reasons before. She should keep testing until either her period shows or she gets a BFP.