Everyone's different, but I relished every "Happy Mothers Day" I got the year Ianto died. I hated when people didn't act like I was a mother, simply because my child wasn't with me.

That said, a lot of people don't like being reminded that they're not spending the special days with their child, so I'd tread carefully and expect a possible backlash if it is said. Maybe acknowledge that it's obviously not going to be a "happy" Mother's Day, but you want her to know you're thinking of her...