There are many V's with this type of structure. Each relationship is its own thing. It sounds like he needs to accept the level of intimancy that he has been given from your other lover for what it is and not expect more. One way to grok it (I love the Heinlein reference) is to realize that every woman has different desires and he should not expect every woman to desire him in the same way.
Of course, the ultimate solution will be to make sure that everyone is communicating about how they feel and what they want. Has she told him how she feels about him or is the communication going through you?