Your friend is lying.
He cannot do this because it's impossible to catch Latios, without migrating from Ruby or Sapphire.
I could trade you Doixes.
You can only recruit legendary Pokemon after you finished the main story(defeat Rayquaza). Even if you did, Legendary Pokemon are usually harder to recruit than normal Pokemon. Some legendary Pokemon have a negative recruitung chance. But don't fret! You can wear a friend bow to increase the chance. The higher your rank is (e.g. Silver Rank, Platinum Rank) the more the likely they will join you. I find Kyogre is the easiest legendary Pokemon to recruit. Some legendary Pokemon can join you by completing specific events. An example is Latios and Latias by rescuing one of them ( I can't remember which) but his event happens randomly.
you ask a friend if they have one
You buy his friend area that has eon Pokemon visit it from Wigglytuff. Then you wait and one morning get Latios and Latias.
What are you offering?
Get one of the legendary birds to join your team.
Legendary island. If you recruit any of the you will get Legendary island.
southern island
Arcanine is not a legendary Pokemon because legendary Pokemon can't be breeded and arcanine can.
Moltres' Friend Area is Legendary Island. You get Legendary Island upon recruiting Moltres, Zapdos, or Articuno.
cause i said so...
You could capture legendary Pokemon but you just can't keep them.