Play the part of leader, do it with conviction. Even though you are just filling in and you may not have officially been handed the baton, that is essentially what you are doing. Hopefully, they will return with their attention and respect. I know it is difficult when the person normally in-charge is absent, you might feel like you are stepping on their shoes if you give too much direction.
You might want to think of it like this: you are playing at a masterclass, the teacher might give some directions and they may be in direct contradiction to what your regular teacher professes, yet some other directions are in agreement with your regular teacher. Nonetheless, you continue on for the short period of the masterclass, following and listening. You do not do any disservice to your regular teacher by listening and trying out another opinion. You might discuss these options at your next regularly scheduled lesson.
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What you describe seems to be a balance issue, therefore an ensemble issue. Personally, I would deal with this first as an ensemble before attempting to single one person out. In a private session the person may not understand nor hear what you are trying to get at. Speak to the ensemble as a whole. Work it out as a whole.
I would outline these points, of course you know this stuff but you may need to reinforce it for the group:
Playing louder does not equal sounding better.
Playing louder because it makes one feel better, does not make a better sounding ensemble.
Parts do not have equal importance: there is a time for each part to lead or to be submissive.
Listing to each other is part of ensemble playing.
Crowding the space occupied by another instrument creates a mess of sound.
Playing louder does not make the listener pay more attention to you.
Get one person in the ensemble to go out into the room to hear how the others are sounding, what the congregation hears. Have each person take their turn at listening from the perspective of the congregation. Have them think about what they are hearing: can one sing to this music, what is the most important part, is that part clear, and so forth. Get them to formulate a response to these questions before you give your analysis. Doesn't matter if their responses are way off, the point is they have begun to think about it.