Thankfully Inigo is used to being teased at this point (thanks, Tidus) that he knows Senku isn't being serious even before the other says he's just kidding. It doesn't even surprise him, nor does it make his smile disappear.

But the next part? That's what gets his cheeks to grow slightly pink.

"Uh.. what?"

He's so baffled.

"Senku, did you really just say that?" Senku? Of all people? Saying he's glad they're friends? Inigo is clearly daydreaming over here, it's the only explanation.