Cheyenne,

Generically, my answer is that it wouldn't but in reality it is really all about the dynamics of their relationship and more importantly the mind of the sub herself. This is especially true if the sub is not bisexual but being involved in this triangle means kneeling to another woman.

Personally, as a switch who owns a sub I have some experience with this type of situation. Although my sub has never seen me play as a sub, I have introduced her to a few of my Dommes and even had her do service for them (not kinky play, more using some real-life skills she has). She has seen how they talk to and treat me as well as how I refer to and treat them even in the private conversations between the sub and myself (I admit that I also use this as teaching moments for her so that she learns by example how to act properly in her role). I know that her seeing me actively kneel in play wouldn't affect her view of me in the least as compared to seeing me jump to get them something in the same way she usually does for me.

From the sub side of things, I know that many of my Dommes are switches or subs in their regular life and have even had conversations about ways to interact with their partner. I have had a couple of them actually decide to switch with me during a session (this never works out for me as I always view them as my Domme even if I am momentarily in charge).

In my mind, the transitive property doesn't even apply as the nature of their relationship with someone else has no bearing on my relationship with the person. Just because my Domme chooses to kneel to another person doesn't mean that I automatically do so unless specifically ordered to by my Domme (in which case, I am still serving my Domme's wishes rather than the 2nd person).

The one situation that your friend should definitely negotiate with any Domme he brings in to play with is about his relationship with his sub. Regardless of the Domme's wishes, he must always be in charge of the well-being of his sub and the Domme must bend to his wishes (without penalty) if he ever objects to any instruction she gives. Also, I would add a hard limit to the Domme bringing his sub into play in a manner to help dominate him (it is natural for the Domme to go for the extra humiliation of having his sub spank or beat him during the session but THAT is something that can definitely muck up the dynamics of their relationship).

Finally, I apologize if others have said some of this earlier in the thread. I am responding to the original post and have not read the other replies before posting this answer. Hope it helps some.