I think you have to take into account the spirit of the rule. It seems to be a common rule even on boards with little moderation. I believe that it should focus on issues like personal information. It's not uncommon for members to privately exchange actual names/phone numbers/email, etc..
If the 'friendship' sours in the future, your new 'enemy' still has your personal information. I understand that PI is already covered in another rule, but it could just as easily be something that shouldn't be shared in public without permission.
And why is there no mention of this rule as it applies to this discussion?
2. Trolling a poster or a thread is against the rules. Trolling is defined by forum leadership as "deliberately offensive or provocative online posting with the aim of upsetting someone or eliciting an angry response from them." Harassment, or the persistent targeting of a member across threads, will not be tolerated. Belittling members will not be tolerated. Calling members out in threads will not be tolerated. This includes doxing a member's private or identifying information. Threats of any kind against any person for any reason will be dealt with harshly and may be referred to a law-enforcement agency as necessary. This includes harassment via PM.
It seems that a member was being harassed via PM. After repeated attempts to have is stop, I now realize that simply reporting the problem is the way to go.