Even if they had a conservatory they could sit in? We know people smoked in there, there was a full ashtray. Why go outside, in the rain/drizzle, in February, when she could sit in warmth while keeping an eye/ear on her toddler? Most smoking mums would not take their kid with them for a smoke, they'd wait until they were engrossed in the TV and sneak out quickly I reckon.
Maybe she didn't want to smoke inside around the child, and/or she was in the habit of smoking outside. I doubt she smoked inside at home.
It doesn't look like torrential rain to me, spitting a bit perhaps. It's a bit hard to say from the footage.