You can also contribute $5,500 pre-tax dollars to an IRA for her, as long as that doesn't exceed the $18,000 limit. Even if she contributes to a 401k, you can contribute $5,500 a year, and I believe that if you're shy of that limit, you can make it up in a pre-tax IRA (instead of a post-tax ROTH).
So if she can contribute $12,500 to her 401k, she can still hit her annual limit of pre-tax retirement contributions.