He us a friend of them. vs He is a friend of theirs.

Some examples from the web:

• 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.
• Eloise Fischer, and to Edward Appleby, a friend of a friend.
• I'm just - I'm a friend of a friend.
• You're a friend of a friend.
• But there's always a friend of a friend.
• She's a friend of a friend of a friend.
• He happens to be a friend of mine.
• Lana, this is a friend of mine.
• Yes, and also a friend of yours.
• Captain Koonan is a friend of mine.
• He's sick... a friend of ours.
• He's a friend of Gengsheng.
• This is Keith Charles, a friend of mine.
• Who is this? - A friend of Carlo.
• I contacted Senator Pete Rayman, he's a friend of mine.
• To see a friend of mine.

Some examples from the web:

• They told them that he is a friend of their father.
• Then those turtles came looking for it, said it belongs to a friend of theirs.
• They won't like us killing these friends of theirs.
• My husband and I are very good friends of theirs.
• It's their friends and friends of their friends.
• All these friends of theirs are sick.
• An enemy of theirs is a friend of ours.
• Hence our older people are likely to end up living distant from their children and the friends of their youth.
• He is a friend of Fulvio.
• Friends don't strip friends of their free will.
• All I know he is a friend of the boss and his wife.
• The guy who wrote this is an individual named Christoph Probst, He is a friend of yours from lnnsbruck.
• But perhaps he is a friend of Mr. Hinckus?
• The one who attacked you is one of their friends.
• Father, Mother and many of their friends came.
• They take care of their friends.
• On the island, The remaining survivors mourned the loss of their friends.
• (Marella) He is a friend of Berlusconi, is a friend of a whole lap that counts.
• And a friend usually knows when their friend is hurting.
• I threw out one of their friends last night.