Put the feistiest cat in his carrier on the floor. Bring pup into the room on a lead and every time he goes towards the cat in an unacceptable way (ie not just to sniff), pull him gently back with a loud "NO"!
And repeat, repeat repeat!
Only do it for 5 minutes at a time to avoid over-stressing the cat and confusing the pup. Once he has the general idea bring him into the room on lead with the cat free (preferably sleeping or he may run and pup will think its fair play to chase). Again, pull back and yell no when he makes to go for the cat.
This is tried and tested by me and some of those who have adopted rescue dogs. In my case I had a year old SBT cross adopted from a pound - he tried to leap into the air to get my cat from my arms, it was a fraught week but he soon got the theory. In time I could leave them together and they were later best of pals.
BTW, one of my cats has practically moved into a neighbours as my current dogs are older now and he is bored of them. Instead he plays with their Lab pup, rolling on the floor and play fighting!
Remember to instruct all the family and visitors to keep the cats and pup apart until you are happy that they are safe together, even if it does mean sticking notices on each door!
You can also reverse the theory by putting pup into a crate and letting the cats wander around the room, once you have tried the first idea.
Finally, all credit to you for being such a lovely family and helping the Guide Dogs organisation. i hope you have a wonderful time with your friend. Don't know if I could do it, I'd sob buckets when it was time for him to leave!