What does the contract say? You could be a hardazz and say not responsible...act of god...bla bla bla, or cough up a couple bucks and 5 min in seed to keep a happy customer, if they are worth it. Depends on your situation.
They agreed to pay for an application of product in the hope that it would grow for them, given whatever weather would come. You agreed to spread the seed, maybe you guaranteed it would grow or not, I dont know. Easy fix for a customer you might want to keep. What are they worth to you?
Throw down a smoke screen....I mean some seed. I have a slope on my yard that I reseeded about 2 weeks ago, the rains came (6") and there was some washout but a lot of the seed is starting to germinate. Just because you see some seed that washed out doesn't necessarily mean that all the seed did.
Had this happen only once...it was for a friend of mine. I had already given him a big break in price and fore-warned him that it was going to rain. I gave him advice on how to fix and sold him some seed at my cost, but that was it.
In your case, I would probably want to help make it right, but there would be a charge involved.
but nosmo.....if he agreed, and assuming he understood, before signing, that we aren't responsible for acts of god, why would he expect me, to take my hard earned money, from my wallet, to seed HIS LAWN? and why, if i refuse, should i be at risk of losing the client? afterall, you heard geo, we are so poor, my children recieve free meals at school. can't a poor man get a break around here?