He heard Yelena's approach before he saw her, a fast moving speck in the sky rocketing toward New Asgard from the facility. If he hadn't given the sight a closer look, he might have mistaken her for his cousin, until she landed of course. Though calling the crash a landing was being a little generous. Rushing to his friend's aid, Clark helped Yelena free herself from the wreckage, and hastily effected a repair on the fence before the more adventurous lambs decided to escape, again.
"Are you alright? What happened?" He probably should have been more surprised by Yelena's sudden new abilities, but all things considered, having his friend suddenly flight capable wasn't all that startling. At least she wasn't mooing.