What time of the year do people go for holidays in Bulgaria, they have sun for how many weeks, competition up against your rental apartment? Who is going to get your renters and collect deposits, how will they behave, who will hand over keys, etc, who will do the laundry and cleaning, who pays for breakages, etc? There's a water bill to pay, there's electricity to pay, management fees for upkeep, etc, set up bank accounts and direct debits. I'll lay a wager with you that there's thousands of apartments for sale there at that price and lower. With great respect, your spouses friend wants someone else beside him with the same asset to feel comfortable. Do I know anyone with similar in Bulgaria? Yes. Do I know anyone with an apartment in Sunny beach? Yes. How are the apartments doing for them as rentals, etc? You really don't want to know. I'm in no way saying you won't do better. Good luck.
Hope that helps.