If already in Canada you may have to take it to a location where they can visually check the Vin of the trailer . Not sure if they would have to check the boat ser # but I would just phone and check. In Victoria it would ba at the office on Blandshard or possibly the airport for anything large that takes up too much parking space as it is limited at the in town office. I had to do this with a piece of equipment I brought up a couple years ago when I decided to license it for the road
He can NOT import the US boat and trailer if it's already in Canada. He can't go get the trailer inspected since the inspection papers and other paperwork is only available at a border customs office during the importation process . As for the certification from the manufacturer stating there are no recalls, it's easy it get. However, if the Vin is faded and you can't read the numbers and you can't locate the hidden Vin sticker, you are going to be in for a huge headache. Once you bring the boat and trailer across, if you are lucky enough to have the Canada customs agent not check the Vin for the trailer, which they are supposed to do, you will have difficulties with Icbc. In order to get insured through Icbc for the trailer, the Icbc agent needs to visually verify the Vin as part of trier checklist for first time registration in BC. Renewals in subsequent years obviously do not need visual inspection. If they can't read the Vin or located the hidden vin, they can't insure during the initial registration with Icbc.
Besides issues with Icbc, you may have issues with the insurance marine boat companies when you are trying to get insurance for your boat if it's not properly imported into Canada as it would still be considered a US boat. Similar to the same reason Icbc would Not be able to insure a car with US registrations and US plates.
Also, if you can't read the Vin or can't locate the hidden vin, the trailer manufacturer can NOT send you a new vin sticker nor can you get one created as this is against the law both in the US and Canada due to the possibility for fraud.
If I were you, I'd look for a boat with BC ownership and registration or look through Craigslist for boats out of WA and OR state... Lots of boats availabke from both those states and the boats are already in the US. Will make the process a lot easier and hassle free for you.