Visitor's visas are NOT VERY easy to come by...MOST people get rejected...ESPECIALLY if they know she has a boyfriend in the US...
They might take this as her way of getting to the country and STAYING here...
Are you in a very committed relationship? Would you be willing to apply for a Fiance Visa or K-1?
If you two can prove that you have an ongoing relationship, with lots of proof, she might be able to be approved for this type of visa...
Once she's here though, you MUST get married within 90 days of entry, in order for her to be able to stay...if not, then she HAS to go back or will be deported...
Visit
Dominicans to the USA - Index for more answers to your questions...
It's the best site for Dominican-Immigration related questions in the world!!!
Good luck!!!
MQ