If your partner is typically more inclined to wash his hair with a body wash-slash-shampoo combination product, actually using up a separate conditioner can help his hair be healthier than ever before so you are starting from its best possible state.
Take this popular Peter Thomas Roth Mega-Rich Conditioner ($18), for example, which is infused with a biotin complex and vitamins C, E, and pro vitamin B5 for soft, shiny hair. Or, there's the nutrient-packed Verb Hydrating Conditioner ($18-$40) with moisturising amino acids plus vegetable keratin protein and quinoa protein to smooth hair and keep it strong.