Bred a dog that was five and 1/2 years old who died this past fall of histiocytic sarcoma. Never had this issue before.

Very, very sad.

Would you offer the owners anything in this case? I'm not really sure what to do.

Well, while it's not "your fault" and crap just happens, if it were me (and I had a similar situation - not this disease, but another weird one that claimed a young dog) - I'd offer them maybe a puppy at half-price, or whatever you felt comfortable with. I'd also make it clear that you know you aren't responsible for what happened, you'd still like to do something to help them out.

JMHO

One of our pups we sold from our earlier foundation girls developed Lymphoma when he was 3 yrs old. They went through all the chemo and radiation repeatedly until their boy died as he celebrated his 4 yr birthday. I promised them a new puppy (free) which they took home 2 weeks before their 4 yr boy passed on.

4 is very young for a dog to die of cancer and though I've never put a clause in my contract about cancer, it's one of those things that I feel compelled to give replacement pups if the family is up to it or any time in the future. Some people need more time to grieve.