Don't decide what to say based on the CBSA report. There are times when IRCC has more information than is available in the CBSA report (at least one forum participant ended up needing to document their presence in Canada extensively, likely as a result of such a discrepancy).
You know when you accompanied your friend, so you know exactly which date and location (and probably even approximate time) this happened. Did any CBSA officer see your PR card or passport on that day? Did any CBP officer see your PR card or passport that day? If so, there is certainly a record of that. If not, there may not be a record.
You mentioned that both CBP and CBSA saw your driver's licence... so there is the possibility they entered that in their system.*
If there is a record then IRCC may have it, and they'll now know you've omitted something from your application.**
If there is no record, they won't know you've omitted something from your application.
My reading of this situation is that if you send an update (using the web form***) to mention that flagpoling (making it clear you did not take a passport, so didn't get a stamp on either side) in the former case they now see you've made an effort to complete your record on file. In the latter case they'll see the same thing, just without the corresponding record.
I think it extremely unlikely that disclosing it would have negative repercussions, whereas not disclosing it seems (somewhat) more likely to have negative repercussions.
But it's up to you to decide what you're comfortable with.
(btw, "negative repercussions" in any case are likely to be requests for additional information/proof of presence, not a denial of your application or something as severe as that.)
* Worst case for these records is if CBSA has an entry, but CBP do not. In that case IRCC cannot know that you did not in fact just spent months in the US, having crossed somewhere illicitly. If that's what happened you are likely to be asked for proof of presence in Canada.
** If they believe this omission makes you unreliable this may trigger a request for additional documentation to prove your presence in Canada. If they believe that all in all your application is reliable, you would be just fine, even with that omission.
*** I'm more cautious than others, so I tend to lean towards using the web form or sending a letter when providing an update, to make sure there's a record of the communication. Mentioning it during the interview may not make it into their notes. It should, but mistakes happen.
