It may or may not. For all the observer knows, it could be a US Citizen that got lost.
If the observer does not know, they making an arrest puts the observed in danger of a arrest himself and being sued.
Two young men did this in Yuma and went to jail. The folks they thought they saw commit a crime, did not and were in fact legally in the US.
A citizens best course of action is to call the BP and follow the person until the BP arrives.
Thanks again for your unbiased opinions and just the facts - that's kinda what I was thinking as well on citizen's arrest (but admittedly, that's not even close to my own practice area, so I appreciate the input).