I also train my girlfriend(we have been dating for years and I recently was certified). I am using a point system promising her a shopping spree for a certain number of points earned. I use process goals(# of workouts, or healthy food sacrifices) and product goals(%body fat reduction, lbs overall weight lost). She is seeing good results after a month or so of training her coworkers are noticing although her overall weight hasn’t changed much she has lost at least 6 pounds of body fat though
I actually train my spouse. However, this is at her request and doesn’t always (more like rarely) work well with couples. I have been hired by other trainers to train with their spouses and family members though. If you go that route: show them how to pick a good one, don’t impose on their trainer (unless absolutely necessary) and be supportive, they need encouragement with impartiality.