Accidental border crossings by organized forces happen all the time, in most every region that has rugged and poorly regulated borders. This is absolutely non-news.
If this was AZ or NM I would agree - the border area on eithe rside looks pretty much the same there. But in this case, the Rio Grande is a pretty large and obvious terrain feature, there are no other rivers out there to get confused with it. I was assuming this is a daylight incident - a night incident would be a different matter (but then how could they claim its a Mexican helo in the first place?)