My DD, also a GK, had this happen to her during CRL in August at Silverlakes. 2nd half about halfway through she vomited at the top of the 18. She continued to play and minutes later vomited again near one of her posts.
Our parents were on the opposite half of the field and never saw either occurrence because both occasions where when the ball was on the other half.
Thankfully we had one of our parents sitting by the corner flag with the opposing team. She ran behind the goal and asked my DD if she was okay and my DD was in a daze. She then immediately called to the ref. My wife was called over, we had a trainer sprint over and my DD was pulled without hesitation.
We got her under a canopy, stripped her down and used cold towels to bring down her body temperature. Once she was in the shade and started to cool down, she began to look a lot like her usual self.
We got her home into an ice bath to bring down her body temperature and she was on a steady hydration cycle the rest of the weekend.
"It'll never happen to us..." isn't an excuse to be dismissive of the consequences and the necessity to have your kids take ownership of how to take care of their bodies before / during / after this events.
Safety first!