Did a quick poke around and I think this is the one he's referring to:
http://search.proquest.com/openview/7f0756636717f6fa2ad91e7fd23230ed/1
"Coached candidates had slightly higher UMAT scores on one of the three sections of the test (non-verbal reasoning), but this difference was not significant after controlling for Universities Admission Index, sex and age."
There are some others as well on that and similar topics, can search English results for "UMAT" on Google Scholar
Honestly I think it's personal. What might help someone might not help another. I know people who got 100th percentile without coaching, and people that got 8th with coaching. I know people who said it helped them and people who said it didn't. Everyone's different, you've got to know yourself.