taking up cycling as a hobby is an option. if you're in the netherlands you should be able to get some interesting places by doing it regularly. having visited stuff in your neighborhood already is an incentive to increase the length of your trips gradually. plan out the trips with destination in mind to fight the tedium.
but yeah, picking up the healthy habits yourself is probably a good way to be supportive; make her feel like she isn't in this alone. and you might be able to get rid of some internal fat yourself. not all of it is visible from the outside and many skinny people carry excessive fat within them.