Mark,
I have two questions and three points to make.
Question 1: Is the Board singling out your friend, or is it suddenly going to uniformly enforce restriction for all commercial vehicles parked on the property?
Question 2: You say the Board has not enforced this rule for several years. Has it been as many as 5 years?
Point 1: The Board is skating on thin ice if it's singling out one homeowner for violation, while deliberately allowing other homeowners to skate. This is illegal, but hard to prove. According to Davis-Sterling, Boards must be even-handed and consistent in their enforcement.
Point 2: According to California Civil Code, if the restriction has not been enforced for 5 years from the time a violation was noticed, or should have been noticed, the Board may lose the right to enforce it:
Statute of Limitations Boards must timely enforce violations of the association's governing documents, otherwise they can lose the right to bring an action to enforce a particular violation.
The statute of limitations for violation of a CC&R provision or a rule is 5 years from the time the board discovers the violation or, through the exercise of reasonable diligence, should have discovered the violation.
Code Civ. Proc. §336(b); Pacific Hills HOA v. Prun. Point 3: If none of the above applies, or if there are no other mitigating circumstances, then we're left with the fact that this restriction against parking commercial vehicles was (I assume) in place when your friend bought into the association. Serving on committees or even as a Board member does not grant anyone any special treatment. (It shouldn't, anyway.) I like your idea of proposing that the Board work with the homeowners to possibly modify the rules to allow commercial vehicles to park on the property under certain conditions, perhaps based on size or in a special parking area set aside for that purpose.
If you want to go to bat for all of the commercial van owners who will benefit from your advocacy, then your friend's fortunes rise or fall on your campaign to find a solution palatable to everyone. And that's your job, right? If so, yiou don't need to recuse yourself from deliberations or a vote.
My 2 cents.
Rob