NFL Prodigy Sammie Coates -- Adorable NFL Advice ... From 12-Year-old Cancer Patient

If Sammie Coates hopes to make it in the NFL, he better take the advice of his BFF ... who happens to be a really smart, really brave 12-year-old girl.

As we previously reported, Coates has become very close with Kenzie Ray -- who's been ferociously battling leukemia -- and even customized his NFL Combine cleats with her name across the front.

So, how did Kenzie react to the gesture? We spoke to K.R. and her mom Keisha on "TMZ Hollywood Sports" ... and it's just one of the best stories ever.

But the best part ... Kenzie's got some sage words for her NFL-bound pal -- and it's just awesome.