My experiences has been, and confirmed here on Trulia, that we are way to quick to through other agents under the bus. The overwhelming patterns is that anyone practicing a business model foreign to us or not meeting the personally approved standard is INCOMPETENT!
As all those before me have written, we've all been there.
The real professional simply does what needs to be done. Sometimes that means overseeing both sides.
When you choose to intervene into a situation into which you are not invited you WILL be the one sacrificed.
Put that agent on your "LIST" and maintain assured clear distance. Let them complain that you don't return their calls.
If you say something to the clients, they'll feel stupid for choosing that realtor and the realtor will probably accuse you of trying to steal the clients for future business.
I had a transaction with a realtor who I think needed to have her meds adjusted. An example of her bizarre behavior was when she asked for an appointment to do the final walk through. We agreed to the requested time. She screamed that her clients couldn't possibly be there at that time (the time she requested).
Because she shifted very quickly from "Have a blessed day" to "THAT WILL NOT HAPPEN!!!!", she made what should have been a pleasant and trouble free escrow into one that wasn't.
When the buyers became sellers, they listed with the raging agent. I have to assume she treated the clients much differently than anyone else involved in the transaction.
The clients probably never realized that her treatment of others in the transaction made it much less pleasant than it could have been.
Some agents have a weird desire to create problems so they can set themselves up as a hero to their clients. It's a really odd approach.