Because before she is a (insert title here) she is a (insert name here). Its a power exchange she shares with her partner, not anyone else, so in that regard you being a Dom to me makes no more importance than saying you are straight. I guess the reason I couldn't take your statement at face value is because I value friendship and bond of trust very highly...and it did sound as if you were going behind subs back (because you didn't trust her to mention it on her own) to inform or ask permission from her Dom if you can be friends with his sub.
Personally, I don't think that a couple has to share all of their friends, I don't think its possible or healthy. And I draw the line where he gets to pick or screen or veto my friends.
If you want to meet your friends partner, then you ask her to introduce you. Going off behind her back and you two sorting it out on the level of higher authority, it changes your dynamic with her. To me your duty of friendship goes to the person you are friends with first, and their partner second. I notice, you didn't say nothing about your sub getting a phone call from the other dom in which he informs her of your friendship with his sub?
And what if your friend doesn't want you to be friends with her partner, because she thought she could confide in you without fear of things going back to him?
(of course when I say friendship, thats all I mean "vanilla" friendship)