friends pregnancy test line disappeard

My friend is a week late on Thursday has sore boobs and I told her do a pregnancy test today, she tested and there was a faint line put a torch underneath and could see it I then left couple minutes because our eyes were seeing shapes because of the torch then the line dissapeared but there was a line a faint one but definately a positive line! I'm hoping so much she is as she's wanted this for so long I'm just wondering if this has happend to anyone else? This wasn't with first morning wee either she's going to do another test in morning.