Can you get a dark postive (clearly POSITIVE, BFP!) pregnant test after just a week of sex.

This question is for my bff... she had sex with her baby daddy Saturday... skip 7 days ahead, the next saturday she took a test and BAM HUGE BIG DATK POSITIVE!?

Buuuuut! Before that, she had sex with some guy maybe 3 1/2 or a month before her baby daddy!?!?! Do you think the test came back postive because shes prego by him or her baby daddy? Like when she took that test saturday the positive came out QUICK AND DARK LIKE SHES BEEN PREGNANT FOR AWHILE!