I agree, that using で is probably unnecessary. ~はどう？is what I commonly hear. However based on the fact that Bob, specifically says どこで食べる？ in the first sentence could be the reason why it might be acceptable to say ではどう？ in the response. However if Bob had simply said 今夜、何食べる？ I would not see any particular reason why で should/could be used.
I agree, that using で is
I agree, that using で is probably unnecessary. ~はどう？is what I commonly hear. However based on the fact that Bob, specifically says どこで食べる？ in the first sentence could be the reason why it might be acceptable to say ではどう？ in the response. However if Bob had simply said 今夜、何食べる？ I would not see any particular reason why で should/could be used.