Hopefully your co-worker has experience as a landlord.
It is one of the BIGGEST pains in the a** I have ever experienced.
BUT, it can be quite profitable IN THE LONG RUN. Short term, think taxes, maintenance, depreciation, etc., etc., etc.,.
Also rent is taxed as regular income. Depending on where you live & buy, his plan may work if rents are high, but the $$ left after the mortgage payment is not pure profit (see above).
In fact, at least on paper, he will most likely LOSE $$ in the early years of the mortgage.
And if he is not escrowing $$$ for future manitenance, he is making a major error in his thinking.
But tell him good luck!