Littlegrape I think that's just a myth, if she is pregnant she'll be in her second trimester so a decent home test should still pick up her hcg without fail. But I still think you should go to the doctors anyway as they can refer you for ultrasounds etc.
Take a home pregnancy test!
I would go to the doctors or clinic and ask as many tests won't be able to tell at this stage If you were :)
Definitely take a test! Me and my mom have always had super irregular periods, she once went 8 months without having a period, and wasn't pregnant