Don't count your chickens before they hatch. I can tell you that failing in school is not always looked at by the court as a reason to transfer custody. Excessive absences and tardies aren't either or fake illness.
The BF? She will probably testify that he is not her BF, just a friend and he does not live with her. Been there done that too.
The e-mails probably won't add up to much either.
But, it's worth a consultation with an attorney to discuss if the evidence in hand is enough to make a go at a custody transfer.
I hate to burst your bubble, but I had what looked like a very strong case and it fell through due to one judge who just didn't see things for what they are.
Sadly, my child hasn't slipped through the cracks, the system has shoved this child down into them and totally forgotten to look back on things to see how it is going now. I have no idea how this will turn out in the end, but for now, it doesn't look promising.
So, I'd proceed cautiously and consult with a good family law attorney who has a proven track record of custody reversals.
In my opinion, I felt that moving on the case after the school contacted me was a mistake. I feel that had I sat back and waited, the administration would have pursued legal means of enforcing attendance and perhaps prosecuted my ex thus leading to a different set of circumstances wherein custody would have automatically transferred to me. However, my intervention for the best interests of my child resulted in the mother having time to create a scenario that just didn't exist and it fooled the court.
JMHO - you need to do what you think is right. But, it will be a stressful and costly experience and may not result in the decision you are hoping for.
Good luck.