[gashatfreak]

Whatever you do, do NOT recommend Kyoryuger or Ninninger as an entry series.

Gokaiager's pretty good, although I'd say your friend would be fine with shows like Magiranger or the (currently) ongoing Lupinranger vs Patranger.

ALSO, some Sentai shows have crossover episodes with Kamen Rider, so unless your friend is a fan of Kamen Rider, probably avoid those crossovers to avoid confusion.

Probably ranting too much by this point, but try showing him the Super Hero Getter song so that he could decide. After all, that song pretty much sums up a key point of a previous series in one sentence without giving away too much spoiler details. As much as possible, show him the updated 2016 version so he has more options to choose from.