I have an older respectable American friend who has come in on tourist visa and has extended his stay by 7 days. He is planning to go to KL at the weekend and then come back to Thailand. What are the chances of him being turned away? I appreciate any thoughts on this.

Since he was only able to get a 7 day extension he apparently does not have a tourist visa. He had a 30 day visa exempt entry.

Unless he has done more than his most recent entry he should not have a problem getting another visa exempt entry.