That is a nice scenario, but BC could have done that all on his own, or HC could have sabotaged her own campaign with one lousy interview, without any help from LL.
The salient fact is that BC and LL agreed to meet on the sly, taking the chance of any number of different fallouts, and almost all of the MSM press was in the dark for a couple of days.
At their word, even if only grandkids were discussed during the meeting, it still leaves an appearance of impropriety that at least nominally they both took a very big chance about. The big unmentioned elephant in the jet would have been the investigation. It would have come up in both of their minds at the first awkward pause in the 30 minute conversation and every awkward pause after that until the end. Both knew meeting was against legal conventions (if not rules). both knew the downside risk of aborting HC’s run for the presidency. Additionally LL must have realized that she was putting her reputation in at least some risk, and her professional future at least some downside risk.
I looked at
NY Rules of Professional Conduct
https://www.nysba.org/DownloadAsset.aspx?id=50671
from presumably LL’s state bar.
However, I could not find any particular rule of professional conduct that seemed to apply.
Perhaps there is a rule governing the actions of federal AGs?
US DOJ Departmental Ethics Office web page
https://www.justice.gov/jmd/departmental-ethics-office
5 CFR 2635 Standards of ethical conduct for employees of the executive branch
5 CFR 3801 supplemental standards of ethical conduct for government employees
https://www.gpo.gov/fdsys/pkg/CFR-2011-title5-vol3/pdf/CFR-2011-title5-vol3-part3801.pdf
28 CFR 45 employee responsibilities
https://www.law.cornell.edu/cfr/text/28/part-45
EO 12731 principles of ethical conduct for governmental officers and employees