I think it's fine. I haven't seen anything saying it's against the rules. Especially the way you're doing it - you're not saying anything offensive, you're just playing and chatting friendly. If anything, you're warning her you're nearby.
My latest game, I rotated bot lane as Jinx to clear a couple waves and noticed Zed was missing mid. I didn't have the river bush warded, but I guessed he was there. I recalled under tower, and he ran at me to try to dive me but was just too late to stop my recall. He took a few tower hits probably due to being unable to ult on me (I didn't see) but I noticed he was pretty low health and walking back towards the minion wave that was now pushing against him. I fired my ultimate from base, killed him. Typed "/all Karma Zed, karma". Not 2min later my team got an advantage and were pressing for mid turrets and overextended. I was backing off already and fine, but they got a kill or two. Then Zed started trying to run around and get me, he chased too far to get in range to ult me, giving me enough time to let my orianna get in range, she ulted him right as he landed from ult, we blew him up instantly. I think I got him a few more times after that, Pretty sure the comment made him really want to get me, but I knew I was far enough ahead to get him so it really paid off.
Basically, as long as you make it really passive, it should be fine. You can't really get reported for stuff like this - you're not being offensive or rude in any way, just really passive low level trash talk.