I understand YOU are in a difficult position but I disagree that HE is.
He is dad and has as much say on how they are dressed as mum does. Certainly when they are at his he is within his rights to say how he expects them to dress and behave.
My ex and I didn't have this particular issue we're generally similar parenting wise but on a few issues we differed and I backed him up and he backed me up. Dd was perfectly capable of understanding there were different rules at the different houses
Eg she had an earlier "bedtime" at his for a number of reasons. She wasn't expected to go to sleep earlier but to give him and stepmum
more time of an evening child free which as they have a brood I totally understood. Different houses different rules. Your stepdaughter is old enough to understand that
But I get that his ex does sound awkward (and irresponsible frankly!) but we're also not getting her side.
But I have come across the type before.
The type that will say they and their child are "best friends"
Dd and I are friendly don't get me wrong we have a lovely close relationship, same sense of humour and some likes in common, but I am NOT her friend.
I'm the one she comes to when she needs a sense check, or a kick up the behind, or reassurance that actually she CAN achieve what she wants to but I'm not the one she gossips to or discusses what a crazy night out she had etc that's for friends.
Too many parents now want to be friends with their kids, too few actually parent.
Re 16 year old ok so it's not that there aren't boundaries in place for her at all it's that they're adjusted for her age and stage - that makes sense
I EXPLAINED to her why I had certain rules
She had strict rules but she knew the reasons behind them
I also enforced those rules/boundaries there was no wheedling out of things or throwing a tantrum and me giving in that NEVER happened.
I never said a no unless I absolutely meant it and once said it was NEVER changed.