Lichess recently added a concise explanation to its site:
Important points:
- Chess.com uses the Glicko-1 algorithm with an initial rating of 1200.
- Lichess uses the Glicko-2 algorithm with an initial rating of 1500.
- Neither of them uses the Elo algorithm (not ELO - it's not an abbreviation).
- Furthermore, it doesn't make sense to compare the ratings even if both sites used the same system, as they have different player pools.
If we are just talking of abstracted playing strength, the assertion that one site is "easier" than another still doesn't make sense. Elo and Glicko systems all have in common that if you get matched with players with the same rating as you have, they will on average have the same strength. So even if one pool is better overall, you as an individual wouldn't notice as you would be matched to comparably skilled players on both sites.
Unless your friend is a Super GM, of course.