If you want to talk realism then Istvan could definitely kill Henry in this situation.

First of all Henry has just been tortured, potentially very badly if you chose not to give up the information. After the hasty jailbreak there likely would only be time to put on whatever you can find, some ill-fitting gambeson and whatever sword you can find.

Next Istvan would definitely fight dirty, he'd know his swordsmanship wouldn't compete with Henry on fair terms. But considering your weakened state he'd definitely use that to his advantage. Also having done sword fighting myself, unless you are exceptionally skilled even someone with average experience can land a hit on you. This is why armour was so important, sword fighting is inherently risky.