I agree about not lying to the parents, because it's very possible for them to find out that it is a lie. All it takes is one of the people invited to tell that boy, or for their parents to mention it to his parents, and you've just created a far worse situation than if you've been honest in the first place. Honestly tell them that either she only wanted girls at her party or that she wanted to keep it small so was only inviting her closest friends, whichever you think is more honest. It's not uncommon for kids that age to do gender-specific parties, especially if they want to have a sleepover party.
Some kids like having large parties where they invite all the class and everyone they know. Other kids prefer having small parties with only close friends. I think both are valid. Childhood politics is terribly complicated, just as complicated as adult politics but without the benefit of experience. Refusing an invitation will hurt someone's feelings. Not inviting someone may hurt their feelings.
I think it's the parents' job to help their children handle this. They should explain that inviting a person to your party does not obligate them to invite you to theirs and that just because you aren't invited doesn't mean they dislike you. They should be inviting people because they want them at the party, not because they want to go to their party. The problem is, not all parents will help their children handle this properly and will take it as a personal insult against their child.
I remember a few times that I gave a friend a birthday gift even though I wasn't invited to a birthday party (if they had one). I didn't only invite people who invited me to their party, and I didn't expect the people who went to my parties would invite me to theirs. There were a few people that I considered close friends that I never was invited to their birthday parties, I don't know whether they had them or not and I honestly didn't care. I still don't. I find it crass to expect someone to invite you to a party/gathering just because you invited them to one. Is it nice to reciprocate? Yes. But if reciprocating makes you unhappy or uncomfortable or is outside your budget, then that's valid.