Is it possible?
I think it should be possible. The force for the projectile is still held in the bow for the most part, right? The cam and wheels just allow for a different orientation for the device.
The idea of a cam predates the viking age significantly. Miniaturizing it might be a challenge, but let's not discount the skills of the blacksmiths of the age, if the goal is described to them I think they could pull it off. Your guy isn't a HEMA professional, right? I don't think there are many of those. Maybe his day job is mechanical engineering or machining.
Based on this sales pitch, the main benefits are
better weight distribution
slightly longer powerstroke, for size
a stabler and quieter shot
So, this isn't going to produce some crazy armor decimating device. Because the force is still held in the bow, and your guy probably isn't going to bring some massive material science advantage with him, this will just give his team access to bows that they could have designed previously, but they would have been slightly large and awkward to aim.
Why didn't it already exist?
On a per capita basis medieval people spent way more time thinking about killing each other than we do (citation hopefully not needed), so these question always come with the built in: why didn't they invent your guy's new idea for a weapon?
I think it is plausible in this case, though. The main benefits are easier aiming for precise shots. The crossbows they had were mechanically much simpler, which is a huge production and maintenance advantage. For war, we expect the soldiers to mostly be firing in formation anyway, so precise shot accuracy for targeting individuals is not a huge priority. For hunting, bows do just fine. Your market is mostly peasants (nobles too, but they are hunting for sport mostly -- they aren't interested in totally optimizing the productivity of their hunting experience, they won't revolutionize bow design to support a hobby). Miniaturized cams will probably require a skilled blacksmith, so you aren't swimming in the things.
In addition, technology development is largely evolutionary in most cases. I don't think traditional crossbows + cams are enough of an advantage to be mass produced. The weight distribution benefit of changing the orientation of the bow isn't immediately obvious, and the orientation change is a pretty big leap. So, it seems plausible that nobody thought to do it until machinists started seeing the prerequisits everywhere in their day-to-day lives, and one of them happened to be a hunter.
You at least have plausible deniability, here.
What good is it?
I suspect that you'll need to at least get the best blacksmith in a large town/small city on board with this plan. You aren't mass producing these things in every little village across the land. Plus, they are more mechanically complicated. So, I don't think you are arming your whole army with these things. These will be your medieval Designated Marksman's Rifle. Plus, they are all hand crafted by blacksmith unfamiliar with the tech so there will be lots of quality variations, especially at first.
Presumably, your guy is also bringing some modern tactics. Perhaps you can arm a small squad of troops with the bows that randomly ended up highest quality, and use them to sneak up on the enemy camp beforehand and take out the leadership. Your superior range, quietness, and pinpoint accuracy will come in handy here. (also, you can probably copy some other modern features, like the bipod). You can probably also embed a couple of guys with the special bow into your crossbow units to take aimed shots at high value targets during battles. A nice perk of this is that the device isn't so obviously weird that the guy will stick out from afar, so to your enemies it will just feel like a weird coincidence that the guy carrying the signal flag seems to always get shot first thing every battle.