I was like your friend...unless I was pushed or coaxed, Id pay for my membership every month and not go. So...what my bf (now dh) did was have my bag packed for me, and told me we were going at such and such a time...and no matter how tired I was from work or how cranky and irritable I was, he dragged my butt to the gym.
Maybe you can get her dh to pack her bag, then give her a call, and say Im going to the gym at ?? time, and I'll be there at ?? time to pick you up, then end the call. Or if she isnt someone you can do that to, ask her out for coffee, and ask if she wants to be pushed, you'll help. Sometimes we need to be dragged
I hope it works out for your friend