I don't know if a ball python can get sick from eating a sick rat, however I wouldn't risk it as well.
When my boy doesn't eat, he eats live as well, I will usually take the rat back to the pet shop or just go to a field and let the thing go. I don't like killing rats personally, if it's meant for feeding then by all means, but anything else no.
Sorry if this isn't much help, just my 2 cents.
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