Yes, the client may be to able to sue you for causing his injury by negligence, or unreasonable carelessness. You used water that was too hot; they over hot water caused his injury. You may have a viable defense--or least grounds to mitigate or reduce what you could be sued for--by arguing that he assumed the risk voluntarily (by keeping his foot in the too-hot water) or was contributorily negligent (by putting his foot into water he knew was too hot). However, that said, he would seem to have grounds for a lawsuit, and could seek to recover his medical costs (if any), cost of medication or bandages, any lost wages (if he misses work) or other out of pocket costs or losses.