Have you read Elizabeth Pantley's No Cry Sleep Solution? There are a few posts here on it too - in the Nightwakings forum there's info on the Gentle removal plan to try and slowly wean off nursing at night. It DOES work and it is gentle, but it takes a long time. I agree that if she wants to keep the closeness and still co-sleep, she needs to move the breast as far away as possible so baby has to work at nursing overnight. Also introducing a lovey can help bridge that distance. I have the book (somewhere...) - let me know if you want more suggestions or info on this.
The other option I see is that they might just want to tackle all sleeps at once to be really consistent. Move her to a toddler bed and explain how it's a big girl's bed etc., and when she wakes to nurse in the night, offer hugs and comfort, but no breast, and preferably not mom. For going down at naps and night, do a new routine and have dad rock her or something. If they want to more gradually wean the nursing at night/naps, mom can use a timer with chime - a 17 month old should be able to understand that when the chime sounds, it's time to stop nursing. Then rock and put down (using pu/pd if crying) for sleep.