If a man is considering dumping his girlfriend to go pursue somebody else than he should absolutely dump her. It is wrong and untrue for both people to exist in a relationship that is, at its heart, completely false. I don't pass judgement on either person for what they choose to do.
They already have a gal, and it is totally wrong to break up with her to date another gal. Polygyny -- the practice of marrying more than one woman -- is legal in many countries and many religions. Here's what Wikipedia has to say on Hinduism polygamy:
According to Vishnu Smriti, the number of wives is linked to the caste system:
Now a Brāhmaṇa may take four wives in the direct order of the (four) castes;
A Kshatriya, three;.
A Vaishya, two
A Shudra, one only.
In Islam, in its Surat al-Nisa', a Muslim may marry two, three, or four women, but only more than one if he fears he wouldn't be just.
In Xtianity and Judaism, a man may have as many wives as he desires, as long as he takes legally.
However, the boy must ask his girlfriend if he wants another one.
As sad as it is I believe it is more honrable for a buy to break up with a girl because they desire another one. It does not sound right but if one person wants another person, that means the first person does not mean enough to that person to keep them from wandering, and it is right to let the original girl go instead of leading her on.