The SkyTeam partners in Latin America are AR and AM, neither of which would make sense for Peru.

To me, the only possible argument for switching would be to change to FB if the KLM flights earn a lot more status and redeemable miles that way, which I suspect is unlikely. The cost would be loss of upgrades (GUCs/RUCs as well as free elite upgrades or upgrading with miles), which could be huge.

BTW, is Lima considered northern or southern South America for upgrades? Is there D1 service on the Lima routes?

The colleague should familiarize himself with the rules about purchasing upgrades with miles or money on KLM flights and on DL flights from AMS. The good recent news is that DL GUCs can now be used to upgrade on KLM worldwide, apparently on any non award revenue fare class.