Are strikes and raids locked behind the campaign in Destiny 2?

I was helping level my friend in Destiny 2, and since I had the XP buffs for Crucible, we just did Crucible all the way to level 20. He has completed a little bit of the campaign, but not much.

Are strikes/raids in Destiny 2 locked behind campaign progression? Will he need to go back and do certain parts in order to complete things like Strikes, heroic strikes, Nightfall and Leviathan?

Yes many different game modes are locked behind the campaign.

The crucible unlocks first, only a mission or two into the game. Basic strikes and patrol missions unlock later once you have all 3 vanguard mentors reunited. And end game activities such as nightfalls and raids are, as the name suggests, only unlocked after the campaign is completed.

These will have to be done on each of your character slots or redone if you delete one.