It's possible they used her because they needed her apartment to make the dabs. Maybe no parents were home and it was the perfect place for him/her to make them. Do we know how old Source 1 is?
This is only a guess, but since he was living with his mom, I'm assuming he is a teenager. I could be completely wrong because the only thing I've been correct about so far is that I did believe Bedford PD had a suspect early on & were just gathering evidence. Then as the case dragged on, I was beginning to have doubts about that. Don't think I have much talent for sleuthing.