Not all women bleed their first time having sex, even if their hymen is intact.

The hymen can be broken in many other ways, horseback riding, biking, falling, and many more. Some women are born without a hymen.

If a girl says she's a virgin there's no reason not to believe her just because she didn't bleed. Blood is not a sign of virginity.

No she's not. Bleeding does not always happen. Bleeding occurs when the hymen is broken.

The hymen can break for various reasons during childhood.

They're both right. All girls are different so all will react different. Sex is no different.
Its best to believe her that she is a virgin... unless she has a reason to lie; does she?

Incidentally, how old are you?