BFN after AF due?

A close friend of mine has been ttc for about 8 months.
AF was due for her on Thursday, but hasn't arrived yet. She did a HPT yesterday & got a BFN.
Is it common to get a BFN & still be UTD? Or is it late enough now that it should be showing.
She said there wasn't even a faint line, so I'm assuming that she's not pg, but it'd be nice if she did the test in a few days & got the BFP.

Has anyone else done a HPT after AF was due & got a BFN that turned into a BFP?
She has had problems with endo, but after she fell pg with her DD (5) she just ignored her problems thinking she must be ok.
I'm starting to think she's not. That she needs to go be checked out again.

she may of had an anovulatory cycle, where she didnt ovulate and they can usually make you a week late at times. i had one a few cycles ago.

it is very common with endo.

id be advising her to go see her doc, get some bloods done for hormone levels hcg, etc and see whats going on.

it could simply be she ovulated late and isnt due for another day or 2.
or she could be pregnant and her levels are just a bit low and in 2 days it will show up.

Well she's a week late today & is still getting a BFN.
I told her to go to the gp. Hoping she listens.