My friend says the game just isn't difficult enough for him when playing inquisition. Is it possible to mod it in order to add lives (or tickets) like in the battlefield series making it so that if the tickets reach zero your team can no longer respawn?

Basically yes, it's possible. The game/server can take in set_soldier_spawn XML command with faction_id and enabled parameters. A piece of script can be added to track deaths and trigger spawns to become disabled/enabled, thus managing tickets.

Strange question, but can this be faction specific? Can you set it so that some factions have respawns and others don't?