a) is she spending an exorbitant amount of time on her phone as opposed to conversing/interacting with you when together?
b) do you have any reason to suspect she's doing anything more than just conversing and shooting the shit?
if you answered no to both of those questions, then there's no problem with it in my eyes. if you grow increasingly suspicious/bothered by it and feel like it's affecting your relationship in a negative way, just talk about it like adults without any accusatory remarks.
or just do what ipfreely suggested i guess if it's really grinding your gears and you see that it will turn into a much larger problem where one of you is likely to implode, rendering the relationship toxic and pointless.
