Since he jumped on it right away and went through it correctly, I really don't think that he shortened it's life. If it were mine though, I wouldn't run it more than a few hours then get it to operating temperature then shut it off and drain the crankcase while it is still hot. That way any moisture or anything else will be churned up in the oil and will come out. I think after that, the tractor will never know the difference.
My $0.02,