All good answers here, here is what I got to say:

• a lot of things could have resulted in such a thing and it is hard to give a definitive answer to your question without knowing everything I need to know, there may be and probably many privacy holes in your system and by system I mean your OS, software and your accounts settings on the internet
• maybe you both wanna take a look at a summary of discord ToS, you might wanna keep tosdr in your bookmarks it is pretty handy and an awesome resource, anyway, are you convinced now? Ready to switch to a privacy-respecting alternative? Apparently there are some privacy-respecting Discord alternatives but IDK which to recommend as idk anything about them (at 1st I was going to recommend Telegram, matrix and signal but these are not a direct Discord alternative although you can use them to chat with your friend just fine)
• switch to a privacy-respecting browser, I recommend Firefox to fight against the chromium/blink monopoly
• opt out of all types of optional tracking/telemetry in everything you have
• if you are willing to: look into setting up a system-wide ad/tracking blocker something like a PiHole or Adguard home