Well if you friend was wearing Rebock - then there is an easy explanation - they were knockoffs - Reebok. Sorry I could not resist since so many brands you see change a little bit of the name.
Being a shoe hound who has bought too many shoes in Santiago, Santo Domingo and Puerto Plata and anywhere else. I have way too much experience of trying on different shoes.
Most of the brands are the same as in the US when it is 10, 8, 7, and in european sizes when you get to the 30s and 40s. But it is always best to try on the shoes before you buy them.
One thing about the tennis shoes (actually alot of shoes), depending on where you are buying them from you may be purchasing the irregulars that come out of the freezones.