The dungeon finder tool only vaguely says “new-to-the-game-friend must reach a higher level”. It never specifies what level that is.
I think it’s at level 20. And I think you can join Battlegrounds at level 15.
I could be wrong.
They should be able to queue up for dungeons starting at level 15. You both need to be able to qualify for the same dungeons, so if you are too high a level, you won’t be able to queue. In order to get around that you can use the party sync feature to temporarily scale your character down. (The party sync button is at the bottom of the quest log/map when in a party.)
Also, be aware that until they have unlocked Chromie Time (which I believe requires they get to 50 on a character) they will only have access to BfA dungeons in the dungeon finder.
Not in Chromie Time, I can pull up the dungeon finder at level 15, but I’m restricted to doing just one, with no option to do random. A second dungeon will become available after a few levels or so, and the option to do a random appears.
For some reason, I was able to queue random dungeon at level 10. I think I joined cata chromie time.