Physical removal or herbicide treatment are the two primary options, and both are included in the management section of our Poison Ivy page. Ideally, the neighbor would help remove it, since treatment of any parts growing onto your side of the fence won't get rid of it permanently since you can't access the roots of the plant or all of its foliage. The only choice if they won't collaborate to get rid of it is to try to put in place a sturdier barrier to its ability to creep through the fence or root under it into your yard.

Miri