As per Islam they are correct in not allowing you to marry her, although they are wrong in saying a (Muslim) 'Bohra' man may marry a Hindu girl. Perhaps in their cultural 'Bohra' customs this is acceptable, however Islam only permits its male adherents from marrying People of The Book (′Ahl al-Kitāb). People of The Book refers to Muslim, Jewish, and Christian women. On the other hand, Muslim women are only permitted to marry Muslim men.
The only way to make this marriage acceptable according to the religion of your girlfriend would be for you to sincerely convert to Islam.
Granted, this may not change the opinions of your girlfriend's family, nor bode well with their cultural norms. That will be up to you to work through. But from an Islamic perspective, if you become a Muslim, then her family would have to come up with a very good reason to prevent her from marrying you (e.g. dificiencies in your character).
I would also like to add that Islam forbids parents from forcing a marriage upon their children. So from the perspective of their religion (Islam), they should not be forcing her to marry a specific person.
I wish you both the best of luck, and hope this answers your quesion.