1. Tell them to stop mowing the lawn. If she can go on a vacation, she can pay teenage boys for their hard work.
2. I personally would confront the neighbor on it. If someone was screwing my boys out of that kind of money, I would be pissed. Be cordial about it, but don't step down from making sure that she pays.
3. You say she is renting right? If your boys aren't mowing her lawn, and she is not keeping it up on her own, that is grounds for eviction from the landlord. The other idea would be to go directly to the Landlord and tell him what is going on as well. That might be grounds for eviction as well.

I hate situations like this. I have a neighbor with a teenage boy who's wisdom teeth were infected and needed to be pulled. I know they don't have a lot of money, so we arranged a 3 month payment plan. 6 months later, I still haven't received any payment. They've given me the run around, and every month "a payment is coming." But I am their Young Mens advisor and don't really feel like sending the family to collections. Working with neighbors sucks.