negative01

As a person who has recent personal experience with the embassy in BKK, I would agree 100% with Ning. The day I was there with my fiancee for her interview, a guy from the U.S. and his Thai girlfriend were there trying to get a tourist visa for her. I knew she would be denied based on the qualifying criteria the embassy uses, but they paid the application fee anyway, and were denied within 1 hour. Why waste your money? You won't get her a visa. Just the fact she has an American boyfriend practically guarantees an immediate denial.

Better make a photocopy of your passport... They will undoubtedly ask for one.

And yes, I agree that your case will be scrutinized especially by US Embassy Manila (if it ever reaches that far).
Thru the consul's eyes, you have the burden to prove that you entered your first marriage in good faith:
1. Pinoy guy has a USC sister living and working in the US. Sibling petitions for Philippines takes 24 years to complete so filing this is fruitless.
2. You meet a USC guy who's willing to marry you.
3. You immigrate to the US (via K1 filed by USC guy perhaps???) and marry.
4. Pinoy guy "patiently" waits in the Philippines.
5. You get your GC and US citizenship then file divorce.
6. You marry Pinoy guy. While you're unemployed, Pinoy guy's USC sister comes to the rescue as joint sponsor for the affidavit of support.
7. Pinoy guy gets visa and GC and you all live happily ever after in the US.
This is how it will look to the consul. I'm not saying that's what happened but be prepared nonetheless.

Exactly. They scrutinize these cases very closely. In their eyes it looks like you wanted to live in the US with your boyfriend and kids, so you married an American, waited til you got your GC/citizenship, then got divorced to file for your ex. I'm not saying that was your intent, I'm saying that's how they will look at it until satisfied that your marriage to the USC was in good faith.

You have the correct answer to your question, must be free to marry at time of petition submission with documentation to back it up.
I might also advise being cautious with this. Something just seems off about her whole situation to me. Just make sure 2+2=4 before moving forward.