Give her a night off
Cook extra when you make dinner, then deliver a couple of plates to her house, with a note that says, simply: "Enjoy!"
Drop by when her hubby's home and take her out somewhere. (It doesn't have to be fancy; a cup of coffee at Starbucks sans kids will be appreciated.)
Take her kid
Babysit so she can go on a worry-free date with her husband.
Tell her you love her
No BFF will ever get tired of hearing it.