In a spirit of Christian generosity and love, let's take the question at face value and educate her without vilifying her
Oh if must (hot and grumpy). No there isn't a 'set' dress code. I've never been instructed to 'dress modestly' or in any other way tbh.
People tend to wear whatever they would usually (maybe a bit smarter/ironed for a sunday, or extra jumper if the heating is rubbish). I've seen pics from the 60s with women in mini skirts in church.
Catholic women will wear a headcover /lacey headcarf if going to visit the pope or other high ranking bod.
Protestant old fashioned types would wear 'sunday best' for church (ie smart outfit, probably matching hat/handbag/shoes). Men remove hats before going into church/cathedral.
Best not to flash thighs or wear a boob tube in church, as you probay wouldn't if visiting great aunt Agatha.
I'm sure in ye olden days strutting into mass or the confesional wearing a tight red silk low-cut dress would have been tut-tutted at, but not a 'crime' in the eye of the church. Wasn't Mary Magdelane a prostitute after all?
The vicar may not judge your clothes, but a coven of congreational busy bodies may well (but that's not restricted to chirch attire, is it?).