I will agree with the idea that you should not broadcast that your partner is new. You need to step in and correct when warranted, and if I sense the patient is nervous about that, I usually make a joke and say yes, this person is new but has been well trained, but I have more than enough experience for the both of us. I'll make a joke about my grey hairs(I earned every one of them) and usually the patient calms down.
If the person picks a less than optimal IV site, for example, (unless its location, size, etc is vital for pushing certain meds) I let it go and address it after the call. As was mentioned, there is a proper time and place to go over details of a call and the actions of a student or newbie. I usually ask THEM how they think the call went, what went well, what they could have done different or better, and more importantly, WHY. It's most effective if a student discovers/realizes mistakes on their own- the lesson tends to sink in much quicker. To me, that says they know what to do, it's just a matter of putting it all together in the proper context.