Ofcourse there are problems but always a solution.
I'd have more faith in his prospects if he 1) he had run a similar successful business in the past for at least five years, 2) had ABUNDANT capital, 3) had lived in Poland for at least three years, and 4) spoke fluent Polish. Or if he had an active Polish partner with similar experience.
As for convenience stores, there sure are a lot of them around. It's going to take some work finding an underserved location that is capable of bringing a profit in. Like you said, restaurants are worse. Lots of them open up and close within a year, predominantly the small ones with limited capital. A farm, of course, is not an option for a non-EU citizen. And the doner kebab factory market is probably already well taken care of. The demand for kebabs is going down, too, as that fad is past its prime and the market is thing out. Again, it's going to be hard to find an underserved market that could bring in a profit.
Terri is right. If you want to make it as a foreign small business owner in Poland, you have to find a niche and create a market. You're not going to get very far aping scads of native Polish entrepreneurs on their own turf. It's going to take a good bit of originality. Based on the types of businesses this guy listed, it seems that originality is not his strong point.