How to permanently delete pictures from all devices in Apple ecosystem?
The answer to your question in the title is one thing, but then there's the question within your post:


  Is there any way for me to be certain of that?


For all practical purposes that question can really only be answered as a no. To be clear, you can never be 100% certain.

However, the closest you can get (without him losing all his other data*) is to arrange a get together with him at his home (without any indication as to why) and then be there with him as he:

On his Mac


logs into his Mac and shows you the photos in the Photos app and, while doing so, make a note of some of the filenames of those photos and videos and then sit there as he deletes all of the items you want deleted
shows you his Photos app preferences by going to Photos > Preferences and pay particular attention as to what, if anything, is enabled in terms of iCloud and then disable these
goes into Finder and empties the Trash (or Bin, depending on version of macOS)
also in Finder, get him to press the command + shift + . keys at the same time so that hidden files are visible and then use commandF to search his Mac for the filenames you made note of at Step 1 to see if any had been saved elsewhere and, if so, get him to go to those locations and delete them all before emptying the Trash again
shows you what other user accounts he has on his Mac (this is done by going to Apple > System Preferences > Users &amp; Groups) and, if he has other accounts, getting him to:


log into each of them and show you what is in the Photos app (if anything) and repeat the first four steps above



On his iPhones/iPads


deletes the photos from all his iPhones, iPads etc (including the Recently Deleted album afterwards), and making sure you check any albums he has created (incl. Shared Albums) and checking what, if anything, is in the Hidden album (located at the very bottom of the Albums page just above Recently Deleted
Opens the Setting app on each of his iPhones and taps on his name at top so you can:


see the iCloud email account listed under his name and make a note of these
tap on the iCloud icon and then on the Photos option to see what options are enabled and disable anything that may be a concern (Note: Photo Stream only uploads photos from the last 30 days, so if you haven't done anything in that period you need not be worried about this option)



Back on his Mac


goes back to his Mac and uses Safari (or another web browser) to go to icloud.com to log into each of the iCloud accounts you made note of at Step 7 and:


tapping on Photos to check what photos, if any, are stored there that you need him to delete
tapping on iCloud Drive to check what files, if any, are stored there that you need him to delete

shows you his Time Machine settings at Apple > System Preferences > Time Machine and look to see if a backup has ever been made (e.g. does it list a drive and show the dates of his oldest and latest backups). If he has been using Time Machine, make a note of any drive names listed and get him to


disable Time Machine
use Disk Utility to erase his Time Machine backup drives

shows you his macOS version by going to Apple > About This Mac. If:


it shows macOS Catalina, skip to Step 11
otherwise, get him to open up iTunes and then go to iTunes > Preferences > Devices and delete any backups that are listed and to repeat this for each user account on his Mac

get him to connect his iPhone to his Mac and then:


open up a Finder window and select his iPhone from the list of devices listed in the Sidebar
select the General tab to make the Backups section visible
click on Manage Backups and then proceed to delete any iPhone backups on his computer
repeat these steps for each device and user account on his Mac



IMPORTANT


There are a lot of things to consider, so it's highly likely that I will have missed something in the steps above
Assuming your friend will agree to doing all of the above (and that's a big assumption), this is still no guarantee that all of these photos/videos will be gone. There is no way for you to  verify that he has never made copies of them on other devices (e.g. USB sticks, Dropbox, etc) or even within other apps on his iPhone, Mac, etc. 
There is also no guarantee that he will not try to use data recovery software to recover items from his external drives.


* That is, it is not realistic to expect him to take more extreme measures to give you this certainty as it would involve him losing all his data