I really don't see a reason to just jump right into telling her everything. There's just something about the way she said 'I was a teenager too' that throws me off. I want to suggest being honest, but leave out some of the details, see how much she can fill in for herself, if she does, we can safely bring her in as a confident or adviser or whatever. I do want to put some effort into keeping the masquerade, even if Hitomi's mother already knows about it, I don't see a problem with keeping vague, if for no other reason that it's one less conversation with Kyuubey about maintaining secrecy. I mean if she already knows, she already knows, not our fault. As a matter of fact, it might be possible that Kyuubey might have undone some memory blockage on Momtomi, but that's just a thought.
[] Be honest, but make it vague on if you're actually Magical Girls, or Larping/playing
-[] Hitomi and Madoka can't and don't join in our hunts and missions
-[] Kaoru and the other two are 'magical girls' from out of town, and are visiting
-[] Everything is good right now, and the team is very strong, and everyone are good friends
-[] If she makes it clear she knows what she's talking about, lose the pretenses and talk openly
--[] Ask how she figured it out
--[] Explain that Hitomi has some potential to be a magical girl, but everyone really doesn't want her to
That got a bit longer than I intended it to, but I wanted to get the right tone and cover certain bases that are true and presented in vague enough ways that could be misconstrued as LARPing or Chunnibyo-ing. It's not an amazing cover, but it really doesn't have to be, I'm 80% sure she knows the basic facts, but there's no reason to go into Lich and Witch bomb territory with Mami sitting right next to us.
Also, I just think it would make a funny bit of storytelling if we try to play it off like this. Afterall, sometimes you don't have to play to win, just have fun with the characters and make an interesting story.