Open up CSGO and head to the "Watch" tab.
Click "Your Matches" to find your matches. (Who woulda thought?)
Find the match you want by scrolling through the matches on the left side. Then click "Download" in the top right of the match stats window.

With your match now downloaded, find the highlight you want to show to your friend and click the round on the timeline at the bottom.
When the match loads, quickly press on your keyboard
and then "Pause" on the new window that appears.

In the same new window, look where it says "Tick" and write down the first part of the number next to it. In the case of the picture above, it's "53770".

Hit resume on the box and watch through your highlight.

Pause your demo when you've finished your highlight. Open up your console (default key is "~", console can be enabled in game options) and type, "demo gototick," followed by the number you wrote down earlier.
Hit enter. The demo should go back to where you had it start before.

If you would like to send your friend the demo from there, skip to the next step.

To get the demo to start at the action points, resume the demo (You may have to close the demo window to switch POVs), and watch through your point of view until RIGHT before the action starts. Pause the demo through the demo window and write down the number next to "Tick" (Just as you did at the start)

You can now close the demo window and exit the demo by hitting escape two or three times, and then "Exit to Main Menu," or whatever it is, I'm too lazy to check right now.Go back through the first step, so your screen looks similarly to the one above. This time, hit "Copy Share Link" where it used to say "Download".
Paste the link to your friend over steam chat or Skype or something. In another message, type "demo_gototick," followed by the latest tick number you wrote down."