Gotta be totally honest, my kiddos at that age would drop drawers in front of anyone. And in truth, really, just my opinion, not a huge deal. I sincerely doubt her boyfriend gave two figs about the 5 year old bathing. And that's not a sexual thing but to a very sick few.
BUT. ya. He should not be dipping the child in her clothes into the bath. And if mom took the wet clothes off, he could have been pushed out the door and hit on the way, playfully as you suggest. With this scenario, it seems like an over reaction to be outraged or offended. Sounds innocent.
It's always wise to watch a single woman though with new boyfriends. Occasionally pedophiles do attach to single women with kids (doesn't have to be a daughter). However, it's not the first thing to suspect nor is it something this sounds like. Grooming behavior for molesting is NOT in front of the mom.