I think the best course of action is to just forget about all this and not respond. She's probably over it by today anyway and certainly next weekend when you see her, this will be all over.
Yeah, least said, soonest mended.
So your DH had already sent her a Mother's Day message on Facebook? Is that what his response "check your Facebook messages" means?
If so, she'll get over it, and if it ever comes up again, you guys should just be puzzled: "I sent you a Facebook message. It's just so expensive to call."
Though I will say, I have told my children: I don't really care about a gift or a card. To me those are pretty empty; they're rote gifts. Even a Facebook message is not really all that special.
What I want for Mother's Day is time with my kids. So when they're young, I want to do something with them.
When they're grown, I want them to call and talk to me for half an hour--and not a half hour that I have to pry words out of them!
My DD is away at college. I called her about 3:30pm bcs I needed to know for sure if she was going to be home at a specific time. She said, right away, "Happy Mother's Day! I was going to call you later." So, OK, but I realized at the end of the day, it didn't feel as wonderful as it would have if she'd beaten me to the phone call.
So, maybe DH should call her and just talk to her, Mother's Day or not, international rates or no.