Hi
I think I would do exactly as your partner did - write a letter and include whatever documentation you have to the fact that there is no divorce in the Philippines. It's my understanding that you're the sponsor, so (like PMM suggested) maybe because you're in Canada, they're asking for proof of your divorce because they think you might have married in Canada. So try also sending proof that your marriage was in the Philippines.
I don't know that you can assume that just because there is no cc on the letter, it means they were satisfied with the proof your partner sent.
But if the sponsor is a CC/or PR (If PR would have to be in Canada) there is nothing stopping him or her from obtaining a divorce in Canada, no matter where s/he was married. I think that is what CIC/CHC Manila wants to know.
PMM