@summ
This sounds so similar to something which happened to a colleague (A) of mine. Turns out another colleague (B) had been texting her every night, demanding replies etc. All to talk about work and how everyone is against her and no one helps her, hates their boss etc. (A) was too nice and would reply.
This ended up really affecting (A) but none of us knew until it all kicked off. (B) did something to really hurt (A) in work and tried to (badly) apologise. (A) asked to be left alone for some time and some peace. If that had been it, it would have been fine. (B) couldn't handle this and kept messaging/texting/calling/emailing/writing letters to (A) trying to justify what they did and play the victim.
(A) blocked and stonewalled them for a few weeks but one day when eventually it was just the two of them in the office (B) kicked off saying because (A) wasn't talking to them they were becoming ill, having to see GP, hospital, night terrors and said all their problems were (A)'s fault.
Thankfully (A) kept all evidence of what (B) did and their attempts to contact them and being asked to leave them alone. (A) reported this to management where (B) was disciplined, warned this was harassment and that if (A) wanted to go to the police they would support them. (A) moved desks and says that she feels like a massive weight has been lifted. No pressure to reply to messages when she would rather be with family. Not having to look after (B) at work events as she doesn't want to go on her own.
None of us knew as from the outside they looked to be work friends but (A) is so much happier now. We are all, also, very wary of (B) and make sure to keep a polite and work only relationship.
Please try and nip this in the bud. It sounds like you're doing that. Maintain the professional (work only) relationship. What happened to my colleague was going on for over 5 years and slowly chipped away and broke her down.