Question regarding bfps

This is for a friend of mine. She is about 3 days late with her period. She took a test 3 days ago and it came back negative. She took one last night and it was invalid (the control line didn't even show up.) With her second pee of the morning today she took an equaline digital and it said pregnant. She took a regular one about two hours later (haf also drank a big water in between) and it came back negative. Then she took another regular one about 4 hours later and that one was invalid. Then she took another regular one at about 9:30 pm and that said negative too. She has one more regular one that she is going to try with her first per in the morning. Any thoughts? She's confused cause the digital she took this morning said positive.

Different tests have different sensitivities. Even tests within the same brand/batch can have different sensitivities. A digital positive is promising, so she is likely pregnant. But she should of course confirm it with another test (digital if possible) or even a beta hcg test in a couple days.

Well she has a doctors appt Monday morning that she is going to no matter what the test says tomorrow morning but we were just trying to figure out if she maybe got a false positive. I know they are rare but can't find any info on the Internet about digital false positives