Okay, that was stupid on so many level as for that senpai excuses to do stuff. She was worried about Chidori being lonely, so she's telling her obvious friend she's her ex-gf to "help" her ? How is that supposed to work, in her mind ? Even if Chidori and Nanoha weren't dating, Nanoha could have had feelings for her and that declaration might have put her off or they're just friend but not everybody is okay with lesbians, sadly, so that could have started rumors about her and isolate Chidori even more.
So yeah, calling massive bullshit on that senpai, which seems in line with how she was portrayed so far, until that last "oh, I'm not being a bitch, just trying to be nice !" act