Of course it is. Don't you do that in Scotland?
Ask him on what basis he prescribes it to be wrong. Find examples from good British authors; (gutenberg.com) is good for this. Demonstrate to him that almost everyone does it; and then tell him that grammar follows language use, not vice versa, and is not set in stone.I do, but my colleague insists it's wrong.
As an example, when I put the question "What country does he come from?" in a reading test, my colleague insisted it must be "From which country does he come?" to be correct.
I'm tearing my hair out to get him to back down. Since we're co-teaching the same stuff we need to agree on tests etc.