There is no set rules in any oh these brother as every brother is different and each to his own and many WLs has their own various circumstances too, basically there is no rule IMHO !
But basic human decency and respects and generousity will always go a long way !
Enjoy it while you can, either inside the shop or the dating outside !
By the way, when you said casual relationship does that mean sexually with no money involved ?
If so, then may be best Not to see her in the shop as it may complicate things !
If the casual relationship doesn't involve sex than no harm keep seeing her in the shop until you move up to the next level !
Again, only the two persons involved here can be the best judge on their own affairs , just my two cents.
Cheers