We got this from friend as a second hand and I wanted to ask if we can take this in flight?
With a small child, you can take almost any stroller to the airplane. Just not as cabin luggage (with you into the cabin), that is reserved for the smallest models, everything bigger than the requirements (this changes from airline to airline, but can be about ~56 x 45 x 25 cm, which this stroller wouldn't meet. everything bigger than those ultra-compacts, including the High Trek, needs be either checked in, or with some airlines, taken as "baby's luggage" up to the airplane, then taken by the stewards and returned after landing.
So you need to check with your airline of choice whether you need to check the stroller in (and how much that will cost), or if there's some baby luggage policy and whether that applies to you (if could be free or cheaper with that option). But overall, yes, you can fly with this, just not as with a cabin lugagge, and the ways to transport it in a plane need to be checked with your airline of choice.