In your sentence
Dann hätte er mich ruhig (mal) ein wenig früher retten können!
the "ruhig" (or the "ruhig mal", respectively) adds the information that the person saying this feels to some degree offended or indignated that the person who saved him/her did so relatively late. In other words, the word, as used here, carries the information that the person speaking has negative feelings about the time of when he/she was saved.
In the comments to the original question, there is now an intensive dabate wether this "ruhig" can be classified as used as an adverb, or as a modal particle (Modalpartikel) such as "doch, gar, aber" in certain contexts.
I suppose the categorization of "ruhig" in this sentence into word classes was not your primary concern. However, in order to show the difference, take in contrast this sentence:
Du hättest mich nicht so hektisch retten sollen, du hättest mich ruhig retten sollen!
Du hättest mich ruhig und besonnen retten sollen.
(One could imagine such a sentence to be heard in a lifeguard training.)
In these examples, ruhig is used as an adverb. It specifies the way how the action (retten) is done.
But in your original sentence, the ruhig clearly is a marker for the mood of the speaker (upset about the timing of the action).
Here are more examples of "ruhig" or "ruhig mal" used in this sense:
Ihr könntet ruhig mal leiser sein!
Du könntest ruhig mal deine Hausaufgaben machen!
Präsident Trump hätte ruhig mal aufs Twittern verzichten können.
Du hättest dich ruhig waschen können.