In the few rare cases where we've seen this, it was because the Mac didn't have the font Arial installed. This is a standard system font and is generally present unless the user has removed it.

Please first check in the Font Book application and check that Arial has not been removed or disabled.

Also, what language or locale is the Mac set to? We have seen some issues with some foreign languages.

You might also try rebooting the Mac just in case that resolves the problem.