They shared the prize money with a friend of theirs.theirstheirs /ðeəz $ ðerz/ ●●● S3 pronoun [possessive form of ‘they’] 
When our washing machine broke, our neighbours let us use theirs. Our house is number 25, and theirs is just opposite. We compared scores and found that theirs were higher than ours.of theirs They shared the prize money with a friend of theirs.2used when talking about someone who may be male or female, to avoid saying ‘his or hers’: Everyone wants what is theirs by right. → his(2)
COLLOCATIONS FROM THE ENTRY They shared the prize money with a friend of theirs.
1used to refer to something that belongs to or is connected with people that have already been mentioned: They shared the prize money with a friend of theirs. When our washing machine broke, our neighbours let us use theirs. Our house is number 25, and theirs is just opposite. We compared scores and found that theirs were higher than ours.of theirs They shared the prize money with a friend of theirs.2used when talking about someone who may be male or female, to avoid saying ‘his or hers’: Everyone wants what is theirs by right. → his(2)