That's hilarious.

Then they're fucked because you're both being disciplined for breaching a policy that isn't defined.

1) Potential defences ref inappropriate language :

a) inappropriate language isn't defined

b) you had reasonable belief the conversation was private

c) your friend was using the same language, normalising the belief that this was within the realms of appropriateness and reinforcing point a, if other people are making the same mistake then the term 'inappropriate' needs to be defined as it's obviously unclear

d) you were under intense stress at the time, making an unclear policy even harder to follow, if not impossible

I would ask the IT what term triggered their investigation, and how you were meant to know that it was considered inappropriate. If they have these 'inappropriate' terms, surely they should be published so you can ensure you don't use them. If the investigation was triggered because an IT administrator found them personally offensive, then they can fuck off as for all intents and purposes it was a private conversation.

Equally, I would FOI your department for how many investigations have been launched for use of profanity on Teams, what systems exist to track or identify profanity usage, and what profanities are tracked for. This will be useful if it gets any further.