1) In Gwent, look at bottom left where you could see *Friends* icon
2) Click on that and it will open the Friends menu.
3) Click on your friend and you will see some 4 options. (Chat, Challenge, See Profile, and ???).
4) Click on Challenge and that is it.
Your friend will get a notification in-game. Upon accepting it and choosing the deck to be played, you can play a friendly game. Also worth mentioning, the coin-flip in friendly match is fixed. Whoever is issuing the challenge will always go first. So, if you want to have alternate coin-flip alternate the person who issues the challenge.