I'm afraid the vet is all backwards on that advice. It is best to wait until rabbits are fixed before attempting to bond them. It would have been ideal to wait until your boy is fixed. Give him up to 8 weeks post neuter for all his hormones to dissipate, and then to introduce him to some other fixed females to check for potential compatibility.
The reason for this is that baby "bonds" don't count since they can easily break apart with the onset of hormones. Babies almost always get along with other rabbits - but this does not mean they will remain that way. It is not a "bond."
Given the situation you are in now, it would be best to keep them apart completely. Not only should you wait until after he is neutered, but it would be best to also wait until after she is spayed as well. It is highly unusual for an experienced vet to loose a rabbit during a routine spay. I would be leery of this vet.
The reason to wait until after she is also spayed is, again, because of those hormones. Once your boy is fixed, his hormones can take up to 8 weeks to dissipate. By then (assuming you neuter him in 2 weeks as mentioned) he will be about 6 1/2 months (an adult), and your girl will be over 4 months. She will be coming hormonal which is not when you will want to be introducing them.
It would be best to wait until after she is spayed and healed, then you can begin the bonding process.
Be sure to read up on bonding processes. Some rabbits simply won't get along. They can fight viciously and even to death. Never put two rabbits in the same cage together until they have been thoroughly bonded.
Right now, any interaction they have will have no positive effect on their future bond. It's those hormones that make all the difference. So even if they seem to get along right now, they can still turn on each other with the onset of hormones. If they do, then their current interaction can have a negative effect because they may remember fights and aggression. So there really is no advantage to allowing them to interact before they are both fixed.