I don't know anything about how she is disciplined or even if she is at home, but you're right that it's unacceptable for your son to be hurt like that. Poor thing. I would not focus on the fact that she's a girl, because I teach my kids they can defend themselves if someone hurts them, boy or girl or even adult. But focus on the fact that she's still a baby, and she hasn't been taught not to hurt people or taught consequences.
15 months seems young for spanking, and young for time out too. My youngest son is 19 months and he is wild too. He climbs stuff, is into everything, sometimes will hit, and his new tantrum is to slam his head into stuff when he is frustrated. It's HARD! I don't spank my kids.. but honestly I never had to with my older boys. Other forms of discipline worked on them. This little one is much tougher to deal with though.. so we will see how it goes. My belief with spanking is it should be a last resort if all other discipline is not working.
Right now, he gets told no, and if he does not listen I will physically move him away and try to redirect him. If he goes back to try it again, then he goes in his playpen for a time out.