The girlfriend may not get in trouble, but the player could/would.
Here’s a 6 minute video that Nelson should have watched in light of missing the meeting. At 2:45, it talks about players sharing “non-public information.”
Not saying Nelson got any “non-public information,” but it seems logical to try to prevent that in general, especially if the question is why “should” it be a rule. The NFL for example has similar rules regarding non-public information - https://www.profootballnetwork.com/nfl-gambling-policy/