I assume he know what he is doing? Sections of the lakes could be dangerous if you don't and end up swimming around in the middle of a loch.
It is generally easy though. My wife and I did it last year. We know a bit about paddling though having done a significant amount over the years (Whitewater, Sea, polo, slalom, WW racing, open boating etc.) But they take groups of kids out to do it as a trip. All that will happen is that it will be slower and harder if you don't know how to paddle efficiently.
Which ever end you start from it is on canals IIRC so the start up period is easy as it is sheltered. Gets trickier on the Lochs if it is windy. We were stuck paddling into the wind, when launched onto Loch Ness there was a few feet of swell and a howling gale. It made for an interesting first hour when we got about a mile and very wet.. So make sure, if you can, that you have the wind behind you.
Good kit. Waterproof coat and trousers are fine for an open boat in bad weather. Shorts and T shirt are good if it is sunny. Sunglasses and a hat as you'll go blind being on the water all day.
For your feet I have found neoprene socks and sandles work best. Sandles if it is nice, socks and sandles if it isn't. It is possible to launch and keep your feet dry but not always. Old trainers for the evening or wellies?
Trainers on the water get wet and stay wet.
Dry bags - sealed well. Critical kit in rubble sacks inside. Having a wet sleeping bag is miserable. Have a small bag for stuff you'll need on the water so you don't need to route around in your main one.
Your stroke uses your whole body, your arms aren't strong enough to keep going all day alone.
Person at the front provides the power. The person at the back steers (and provides power).
Take a small tent - if you are wild camping then this allows more flexibility.
Weight is not much of an issue, don't go mad but you don't need everything to be super light. For food it is easier to have stuff to heat up rather than rehydrate.
There are shops, pubs etc. on the route. It can be remote but it needn't be.
Plenty have done it from here so post up if you have specific questions.