Look for open mixed tournaments.
Rated I guess you guys could do 9.0 (5.0 plus 3.5), but I'm not sure if they would allow such a big difference in rating between the two players. USTA mixed leagues generally don't allow more than 1.0 between partners, so I would just check the rules where you live, and for a particular tournament.
As the female and weaker player, she should expect a lot of balls hit her way, obviously. The two of you will have to work on some way of neutralizing that, especially against two evenly matched opponents. In mixed, it is usually the female that makes the difference.
And you should hit her lots of serves. Tons. Billions even. All kinds. Don't back off. That return of serve is *crucial*!
What would you do if you knew you could not fail?