So right off the bat your scenario doesn't really add up.
I can tell you plenty of guys that weren't good enough for the NBA who went to Europe and then ended up coming back to the NBA and having success.
The jazz have a rookie right now that went undrafted and went to Europe.
There are quite a few guys who have found their way to an NBA team after going undrafted and going to Europe.
It's impossible to say whether staying in college or going pro is better for his pro career at this point. It can be argued either way as there are pros and cons to both.