This sentence is incomplete and therefor wrong. The relative clause (the part after the comma) is correct, but the main clause (which could exist without the relative clause too) is missing a part of its predicate.
To answer your question: The verb of the main clause is »sind« (in English: are), but that doesn't tell you why the sentence is wrong.
The word »sein« (to be) is a copula. Copulas are verbs that do not describe an action, but they describe some sort of binding, connection, linkage or coupling (that is where the name comes from).
German copulas are:
- sein (to be)
Anna ist müde. Otto ist Katholik.
- bleiben (to stay)
Frau Schuster bleibt verschwunden. Herr Mayer bleibt Direktor.
- werden (to become)
Die Kinder werden älter. Irene wird Rennfahrerin.
- heißen (to be named, to mean)
Ich heiße Hubert. Dass er das weiß, heißt nichts.
Many other words also can used as copulas although the are normally not listed in lists of copulas.
Maybe the most copula-ish verb of all is sein (to be): Something is something else. And in your sentence the part something else is missing. What are her or their friends. (Without any context you can't tell from this sentence if »ihre« refers to a single female person or to many persons.)
Given (main clause only):
Ab und zu sind sogar ihre Freunde.
Every now and then, even her/their friends are.
What are her/their friends?
A possible correction:
Ab und zu sind sogar ihre Freunde gemein.
Every now and then, even her/their friends are mean.
Here I used the adjective »gemein« (mean, rude) to complete the sentence. But, as shown in the examples above, the grammar also allows you to use a noun or a nominal group. But it's hard to find a noun or a nominal group that semantically makes sense in this concrete sentence, but here is an example to show that also nominal groups work (at least grammatically):
Ab und zu sind sogar ihre Freunde kleine Kätzchen.
Every now and then, even her/their friends are little kittens.
Another way to correct your sentence, is to interpret ihre Freunde as the second part of the coupling, so we have to find the missing first part:
Ab und zu sind sogar Hunde ihre Freunde.
Every now and then, even dogs are her/their friends.
Ab und zu sind sogar Leute ihre Freunde.
Every now and then, even people are her/their friends.
And in the last example a relative clasue, that gives further information about those people makes much sense:
Ab und zu sind sogar Leute, die sie wegen ihrer Gutherzigkeit ausnutzen, ihre Freunde.
Every now and then, even people who take advantage of her/them for her/their goodness are her/their friends.
You can also move this relative clause to the end of the sentence:
Ab und zu sind sogar Leute ihre Freunde, die sie wegen ihrer Gutherzigkeit ausnutzen.
Every now and then, even people are her/their friends who take advantage of her/them for her/their goodness.
Another way to express the meaning from the last sentence is this:
Ab und zu nutzen sie sogar ihre Freunde wegen ihrer Gutherzigkeit aus.
Every now and then even her/their friends take advantage of her/their goodness.
But this is a completely different grammatical construction. To turn this sentence into the structure of the sentences above you need am expletive subject, i.e. you have to use the word »es« (English: it) as the subject of the main clause, and then the relative clause describes what this expletive subject stands for:
Ab und zu sind es sogar ihre Freunde, die sie wegen ihrer Gutherzigkeit ausnutzen.
Now and then, it are even her/their friends who take advantage of her/them for her/their goodness.
So, as always, when there is an error, that are many different ways to find a correct version. Only the context of the story, in which this sentence is embedded, provides hints for selecting the best correction.