Sorry to hear that you lost your mom not too long ago.
Koodo's customer service is giving you the run around. Once you were able to prove to Koodo that your mom died, they should have allowed you to change the PIN on the account and granted you full access. This is a necessary step as a measure to prevent people from stealing other people's accounts and compromising them. The fact that they were able to cancel the account for you means that they are satisfied that you are authorised to do so. So them now saying they can't discuss any charges makes zero logical sense.
Like it or not, cell phone companies do have a 30 day notice of cancellation clause that says that they will charge you for up to another 30 days after you've requested the cancellation. They don't always charge this fee and sometimes even refund for parts of that last 30 days. The Koodo terms of service will surely state that they reserve the right to charge for another 30 days, and it appears that's exactly what they have done. Pretty much they are saying since that no prior notice of cancellation was given that service will remain active for another month (until June 17th) and that they will charge for it.
But just because something is stated in Koodo's term's of service, doesn't make it legal. If you are certain that the CRTC ruled that cell phone companies can't charge for 30 days notice after the death of a customer, file a complaint with the CCTS. They handle complaints within the industry.
Another option is to just make sure that Koodo doesn't get paid by reversing the charges and blocking all future payments from the bank account/creidt card. Koodo will eventually send the amount owing to collections but it's not as if your mom's credit rating could be harmed. It's a matter of how strongly you feel about Koodo getting paid and whether such measures would be worth your trouble.
If the charges were being taken from the bank acccount/credit card on May 14th, that means that the charges for an additional 30 days after May 17th would have been mostly covered already on the May bill. This depends on the Koodo account billing date. A few days out of that 30 would be billed on the June bill. To me, it doesn't make sense that you were told that the 17th was the last day of each billing cycle. This is because preauthorized payments came out on the 12th and 14th of different months. There's no way Koodo could have possibly knew how much to take as a payment before the bill was even generated for each of those months.