I would never BB any WG, regardless of gender, to do so is frankly insane.
PrEP is actually very effective against HIV but would you trust a WG when she says she's taking it? Is she taking it properly? Did she get it over the internet or from a trusted source? Just not worth the risk.
The other thing is of course, PrEP will only protect against HIV, not against any other STIs. You can catch pretty well any STI from anal sex, exactly the same as from vaginal sex (perhaps even more?) and the likelihood of doing so is probably increased from even small rectal tears, something which doesn't happen with a vagina, which is designed to take a cock.
So again, just not worth the risk.