In this case the protection isn't the significant point, peace of mind is and if she needs to pay for it so be it. I've been there several times before, people ask your advice, you explain carefully how avast! and AVG are both really good commercial software packages and the protection on the free one is the same as the paid one. But there eyes have already glazed over and you know they stopped listening the moment your suggestion didn't match what they'd already decided to do, which was update the ninety day sample the PC came with, after all if it's free it can't be any good.
Think calming mellow thoughts, let it wash over you and remember next time they ask for advice or help, there really is no point in giving it.
Why not get her to buy the pro version of avast! That way she's happy and alwil have a bit more money in the kitty, helping to perpetuate my favourite freebie.