Is is the present-tense form of wees, which means to be. In this sentence it links the subject Sy to the description lief vir haar troeteldier.
One helpful thing for learners is that Afrikaans verbs usually change less than English verbs do. With wees, you get:
So is is very common and easy to recognize.
Yes, it is one word. Afrikaans very often forms compound nouns by joining words together.
Troeteldier is a compound:
So troeteldier literally has the idea of a pampered animal, which is why it means pet.
The plural is troeteldiere.