Does this phrase refer to the looker's look or the lookee's look?

If "lookee" is the one who is looked at, then the lookee's.
По одному его взгляду было легко догадаться, что он расстроен.

Aha!

By the way -ee is very productive (you can use it with any verb) and it’s quite common to use this when you clarify things like this.

I don’t like either of your translations. We say : JUST ONE LOOK (and you knew/made it clear/and there was no doubt, etc.)

Just to invent any word, right?

Yes. It’s kind of colloquial, so it can be humorous too: “Standing under the mistletoe can be a risky business unless both the kisser and the kissee observe good oral hygiene.”

ок)

👍🏼

😘🤮