Here I wrote some guides that could help him:

• https://darthcoin.substack.com/p/first-steps-to-accept-bitcoin-as
• https://darthcoin.substack.com/p/bitcoin-for-merchants
• https://darthcoin.substack.com/p/lnbits-for-small-merchants
• https://darthcoin.substack.com/p/the-bank-of-lnbits
• https://darthcoin.substack.com/p/operating-with-ln-as-a-merchant
• https://darthcoin.substack.com/p/getting-started-lnbits
• https://darthcoin.substack.com/p/lightning-wallets-comparison

A TLDR:

• start with simple ones and slowly once he will understand what is going on, move to more self-custody.
• the most simple way: coinos.io and move the funds to an onchain wallet that he can control, each day or week, whatever he consider enough to move.
• for a face to face shop, even Phoenix or Breez are the most easiest mobile apps to use for someone that have no idea how to deal with accepting BTC.

You speak spanish, so please watch also this excellent episode from @Lunaticoin
https://www.youtube.com/watch?v=SfzmD504jZ4