What if Snow went to the BF for help to fight against Regina and the BF said that she couldn’t help and Tinkerbell over heard and told Snow that she could help her since the BF wouldn’t.
Or maybe the BF did to Snow what she did to Rumble when he asked the BF if there is another way. Snow could have done the same thing and asked if the BF for help but the BF wouldn’t/couldn’t help Snow, so maybe Snow asked her who could or figured out that there is other fairies that could help her.