So your friend may get away with it for years - he gets no tax form from his “employer”; he fills out his taxes, per instructions on the form, by adding up all the income for which he has forms - which will include tax deducted. Nobody will notice…
Until one day when the building management company is audited. Of course, another possibility is that the building manager wrote a business cheque for “CASH”, pocketed that, and paid your buddy with a personal cheque, specifically so that there would be no trail from the business to buddy. I assume in this scenario if it’s not his business (or even if it is) he pocketed an “expediting fee” in this transaction as unreported income to himself too.
After all, if he uses the “unknowns from craigslist” story, and the units did need cleaning, how likely is it he’ll have to worry - unless he gets greedy and the IRS says “too much for anonymous casual workers” or someone talks.
The problem with “working under the table” is not so much getting away with a payment, it’s getting a regular paycheque. While buddy could get some serious beer money this way, if he starts making a regular living wage, it’s coming from somewhere. Unless the guy manages 10 or 20 building companies and spreads the load around, it will eventually be noticed if he’s shelling out $30,000/year or so. This is why home repair contractors are the ones most able to do this trick. They have hundreds of customers spread all over the place who are generally untraceable.
Another trick the IRS can do in extreme cases, I assume (Canada does it) is an personal assessment. You live in a house that costs $X, you drive a car that costs $Y, you must buy gas and groceries but we see no weekly withdrawals from the acount you put your paycheque in. Evidence points to you having an income of $Z so we assess you the following tax unless you prove us wrong…
I see where the Canadian Revenue Agency, for example, swooped into a St Catherines area restaurant, tallied up all the tips on receipts (good old Visa!) for the last 2 years, and demanded the waiters pay back taxes (and penalties) on that. While often the small fish like your friend are not worth chasing unless they fall into the government’s lap, in this case apparently that audit prompted updated returns from many of the servers in the area.