Rich pensaba que se las estaba arreglando...

[Tyrn]

Hi,

Rich pensaba que se las estaba arreglando muy
bien hasta que comenzaron los vómitos.

What is this las, and why plural?

[aleCcowaN]

Another instance of lexicalized pronouns

arreglárselas = cope, manage, get by (without any aid)

[aleCcowaN]

The original text is "Rich felt like he was doing pretty well until the vomiting started"

Again, problems with the translation. Arreglárselas implies Rich is the agent in a wide variety of situations (he copes, he manages) while be doing well implies he is getting better after an illness.

In my opinion, a more proper translation would be "Rich sintió que estaba mejorando sostenidamente, cuando comenzaron los vómitos" where cuando substitutes until implying the turn of the events