How to handle obsessive friend getting out of hand?

Sorry this is so long! Any advice appreciated! We are a group of 3 friends, all (18f)

2 of us recently met a new girl. I was friendliest with her and she's been normal to me. But my friend, after knowing her 2 days, had this new girl spam calling her etc to meet up. We don't meet up alot, we spend time at college together. She doesn't like texting etc, so she politely declined and ignored until the spamming stopped.

When our 3rd friend met her(III call the new girl Tilly, and the 3rd friend Jacky) She did the same thing as before, but only a day after meeting Jacky. She arranged a sleepover, and spammed her. I went too, and it started normal, but Tilly told me she was bi, and that she wanted to tell Jacky, and nobody else knows. I left the sleepover because I was ill.

After that, Tilly spammed Jacky on all socials. Jacky isnt online much. Tilly told her in college she wanted to hang again just them. Jacky said she was busy, and Tilly said okay but she has BPD so she'd struggle to accept it. Tilly spent the rest of the day begging Jacky to come over anyway, pouting, putting on a baby voice, saying she was 'splitting' from bpd.

Tilly continued to spam Jacky. Jacky had to go into college, when she got there she was told by everyone that Tilly was looking for her and was upset she was ignoring her. Tilly didn't even know that Jacky was going to be in that day, its summer break, so it was a one off. Jacky said she's been busy, and Tilly said it was fine.

Since then, Tilly is still spamming just Jacky to no reply. She posted photos oftagged her in them. She had already posted them at the time, and it was ages ago . And she's sent texts to Jacky that she's near her house / work, even though Jacky barely replies to her messages.

We hoped it would be like before - she would take the hint and stop spamming, however we're worried it's got to the point of obsession? Again, I have been more friendly with her and reply to her messages, but I do not get this, she barely messages me.

EDIT: Since I'm getting some nasty replies calling me stupid and saying it's Al, the post is weird / choppy in places because I cut it alot in places so it wouldn't be too long. Please take 5 seconds to check my profile, ask me in the comments if you'd like to confirm I'm a real person, just don't leave a rude comment! Ty!