Looking on the internet deeply has found these results:

He us a friend of them. is the most popular phrase on the web.

He is a friend of theirs. 1955,000 results on the web	He us a friend of them. 12,260,000,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
They told them that he is a friend of their father. They won't like us killing these friends of theirs. Then those turtles came looking for it, said it belongs to a friend of theirs. All these friends of theirs are sick. My husband and I are very good friends of theirs. It's their friends and friends of their friends. Hence our older people are likely to end up living distant from their children and the friends of their youth. An enemy of theirs is a friend of ours.	I'm a friend of Carlos. Carlos is a friend of a friend. Mr Barroso said he had been on holiday with a friend of 20 years standing, on the yacht of a common friend. Mr Barroso said he had been on holiday with a friend of 20 years standing, on the yacht of a common friend. The friend of a friend delivered. I'm just Eloise Fischer, and to Edward Appleby, a friend of a friend. I'm a friend of a friend.

He is a friend of theirs.

1955,000 results on the web

He us a friend of them.

12,260,000,000 results on the web

More popular!

They told them that he is a friend of their father.
They won't like us killing these friends of theirs.
Then those turtles came looking for it, said it belongs to a friend of theirs.
All these friends of theirs are sick.
My husband and I are very good friends of theirs.
It's their friends and friends of their friends.
Hence our older people are likely to end up living distant from their children and the friends of their youth.
An enemy of theirs is a friend of ours.

I'm a friend of Carlos.
Carlos is a friend of a friend.
Mr Barroso said he had been on holiday with a friend of 20 years standing, on the yacht of a common friend.
Mr Barroso said he had been on holiday with a friend of 20 years standing, on the yacht of a common friend.
The friend of a friend delivered.
I'm just
Eloise Fischer, and to Edward Appleby, a friend of a friend.
I'm a friend of a friend.