Hitting

So my friend has a 2 year old son and a one month old daughter. My daughter is 16 months old (so 6 months younger than her son). He used to only hit if she would take something and we would step in but it didn't happen often. Well since she had her daughter the last few weeks when we get together her son goes straight for my daughter and hits her multiple times, sometimes with items sometimes with his hands. It has been bad the last two weeks and I want to protect my daughter but it's over before I can get to her. But she has started flinching when he gets close to her. How can I handle this without hurting my friend because I know I'd feel bad if the roles were reversed but I gotta protect my kid. (Thankfully she hasn't picked up on hitting from him yet). She does try to step in but is not always consistent in how she approaches it.