She needs to be spayed to rid her of hormones that typically interfere with a bond. Even once she is spayed, however, there is still no guarantee that the two will decide to bond. (Intact rabbits are much less likely to bond period.)
This is the reason it is recommended to let your boy meet some already spayed females to pre-screen for potential compatibility. This greatly increases the chance of a successful bond. Then, if a promising potential happens to not work out, a rabbit rescue will work with you and allow an exchange to help ensure you wind up with an actual bond.
Another bonus of taking this route is that it saves you from a difficult situation. If you instead get an intact female with intentions of bonding her, then get her spayed (costly!!), then wait out the time for her to heal, and have her already well-acclimated to her new home,...if after all of that, the two rabbits decide to not get along, then you're stuck with the newer rabbit and may be forced to house the two separately... for the next 10 years.
You can read more about bonding and bonding options here:
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