Best to prevent a snake from drinking water with soap in it- it's likely to cause stomach distress, & since she already has mites, you don't need any more things causing dehydration -assuming the soap would likely cause some diarrhea, so between that & blood loss, dehydration is serious & can kill snakes.

You're better off doing the water soak with a bit of soap only with your close supervision, for about 20 minutes or so. It won't get all the mites, but will help to drown most of them. You also have to worry about mite eggs quickly hatching out subsequent generations to plague your poor snakes. Mites can also carry diseases, so I hope you're quarantining your snakes apart also.