The "easiest" setup for Lightning on Nostr would be to use the clients with integrated wallets, like Primal and Wisp, which do so using Spark. Moving down the list from there would be custodial Lightning wallet providers that have Nostr Wallet Connect integration. This just requires a brief wallet setup and then copy/pasting a NWC connection string into the user's chosen client. This is probably the method supported by the widest variety of Nostr clients these days. Users can opt to use providers like Coinos, Rizful, or MiniBits, or Npub.cash. Zeus is a bit of a hybrid. It starts out as custodial using ecash for new users who don't yet have enough sats to justify opening a Lightning channel, but it provides a Lightning address for receiving zaps. In my testing, it can be connected to Nostr clients for sending zaps using Nostr Wallet Connect, but this doesn't seem to work with the ecash balance, only once the user has set up a Lightning channel and have a balance available. That being the case, I would recommend one of the above options until they are ready to take on self-custody.