Just use calipers and the pair he has. 25mm tires aren't a new invention of the wider is better trend, they've been around for decades and would have been a normal usage for someone who wanted a more comfortable ride or and heavier and slower training tire. The tire might have a slight lightbulb look but nothing unsafe unless you're pushing the limits of cornering. This is assuming this is an old aluminum version and not something new I'm unaware of.