Non-custodial route, but very easy would be for them to buy some sats on Strike, cashapp, etc and then just send you the sats via LN.

I do this scenario for people that I know pretty well and they trust me, I onboard them first with a lndhub account on my private LN bank. °As I explained here

Will install Zeus or Bluewallet on their devices and just send them some sats to play around. Also if they are too stupid and lose the access to that first BTC/LN wallet, their funds are still in my hands and they do not lose them, so they anytime can come back to me asking to recover.

After that I teach them how to climb the ladder of self-custody with more apps. Important is that you show them how it works.

Option 1:

Hope hierarichal channels are implemented which would allow you to add participants to a channel off-chain. Or wait for eltoo

Option 2: If you have a Lightning channel, you could buy your friend inbound liquidity through some LSP like lnbig.com. But keep in mind lnbig (and other LSPs) seem to charge quite a bit these days because of high on-chain fees. Even a 100,000 sat channel costs $14, when a few months ago I got one with like 500,000 sats for around $3-4.

I would have recommended Phoenix, but to get a large channel you'd have to transfer a fair bit of funds to him first to open the channel, and then transfer it back I suppose

Well the best thing, would be to create something that doesn't exist lol. No one that I know of, between LSPs and Liquidity providers like LNBig does transaction batching. If you batch, the cost per user can be much much less and as a result compete with these higher paying transactions.

So you either need an LSP that says "Please wait a day for your order to complete" to allow for enough time for enough liquidity orders to come through to batch everything together, or a liquidity provider that does the same.

For more information: https://thebitcoinmanual.com/articles/open-multiple-lightning-channels/

I think the future will be custodial for many people. Certainly for small amounts. Barring any major improvement to on-chain scalability, but that's beyond the scope of my expertise.

Having some sats on Liquid is useful as well now that you can swap from Liquid BTC to Lightning https://liquid.boltz.exchange/

have them buy bitcoin on an exchange with LN integration

Well, fees aren't that high when you look at it from a fiat mindset. 20sats/vb is about a dollar.

Custodial is cheap

Non custodial you'll need to pay a bit more