Does Blue Ahri give -2 to Ruin Runner when attacking or defending?

No, Blue Ahri cannot give -2 to Ruin Runner, regardless of whether it is attacking or defending.

According to the card text retrieved for Ruin Runner, it has the ability: "I can't be chosen by enemy spells and abilities."

As established in RiftJudge FAQ #468, Blue Ahri's ability targets the unit it affects. Because Ruin Runner has protection from being chosen by enemy spells and abilities, Blue Ahri's ability cannot select it as a target. Therefore, the effect will not apply to Ruin Runner.