I make my living from rental income and am convinced that -generally - there is no point in trying to make any money from rent if, at the same time ,one is paying off a loan.
I am talking about making some income, not the hope (forlorn in the last several years)that the property may increase in value. That's another aspect as to why many people entered the BTL -hoping that the rent would cover much of the loan and the property would double in XYZ years. Leave that aspect to one side and concentrate on income.....
As regards pure income for people borrowing ;- the real answer is that ,yes, theoretically there is an income as per your own example. Your friend is making an income of 5-6.000 euros BUT friend has to pay a loan PLUS income tax which, together, are more than the theoretical income. And, as I pointed out before, if friend is on higher tax bracket then he will lose a lot of cash flow each year.
The problem for inexperienced renters with large loans is that many do not understand the difference between income and cash-flow. And ,sadly, Revnue don't give tuppence about the cash-flow problem . So, on paper one can make €10k rental income, pay 5k tax plus another 10k back to the bank each year. That's a "loss" each year of 5.000 euros for a lot of hassle. And yet it's an income of 10.000 euros. Get it?
-and this is why so many many second home owners are in trouble. They are making an income and yet finding they can't pay their loans
Other posters will say I'm exagerrating. And, of course, if one has a good income and can afford the tax plus mortage repayments,that's great. But, unless one has a good monthly cash surplus after all other costs then I think they'd be crazy to borrow to BTL.
And,finally, even if one had the cash to buy outright things are getting tougher and tougher for landlords.
You raise a good question and doubtless you'd welcome other contributions to this that may not have such a low opinion of BTL as mine.