I say, and with God's success: His ablution is valid; because the water mixed with used water is judged by the predominance of weight, and here the used water is little compared to the rest of the water. This is unlike someone who performs ablution from a weak-flowing river, who must sit in a position where he does not use his washing water, or he should stay between the two scoops for as long as it takes for his washing water to go away; so that he does not perform ablution with used water, as it is not permissible to do so. Refer to: Sharh al-Wiqayah p. 97, Ghinayat al-Mustamli 1: 819, Al-Bada'i 1: 71, and Fatwas of Qadi Khan 1: 4, and God knows best.