No way to tell what client somebody is using based solely on the UA of their default browser. All you know here is they're on Windows (making, say, Entourage/Apple Mail impossible and some version of Outlook likely). If MUA info were leaked into the browser it would be pretty strange, right?
One thing you can do when you see people doing "View as Web Page" is perhaps send them a personal follow-up question about their email client ("We'd like to ensure compatibility"). All they have to do is reply and you can see their MUA, which is kind of neat. You could even be proactive and have "Need to view as Web Page? Tell us why" in every email.