OK; my opinion, which probably isn't worth anything. As far as schumer goes; I wouldn't contact him about anything. Why? Because he hasn't done anything for the applicant. If he gives him a nomination, then tell him that Senator so-and-so also gave a nomination.
The representative's nomination pulls the most weight. Especially if you are the principal nominee. So if you have to tell the senator (Who gave the nomination) that you already received a nomination from your representative, then do what you have to do. But check their rules carefully. Does it say something like: "When applying for a nomination, inform us if you've already received a nomination". That would imply that you had a nomination PRIOR to applying for theirs; and thus, they don't want to waste one on you if you already have one, and there's a LOT of applicants. However; if at the time that you applied for for nominations, and were interviewed, if you didn't have a nomination from one of the others at that time; I wouldn't mention it to them.
Look; I understand what they are doing. They want to maximize nominations to the academy. Instead of 2 senators and a rep all nominating the same 10 people; they can each nominate 10 different people and effectively have 30 nominations. I understand the game. I've played it long enough. Same thing happened a few years back when my son applied. He had a presidential, but the academy wanted him to get a senator or representative's nomination. This way they could hold onto the presidential for someone else who qualified, and couldn't get their MOC nomination. Again; I understand the game. But guess what????? This is your life. (Or in the OP's case, his girlfriend's son's life). As such, you/he has to look out for what's best for him. If I knew for a fact that my senators and representative would talk to each other; that they would indeed coordinate their efforts; and they DIDN'T RANK their slates; then I would cooperate to "ASSIST" them in maximizing their nominations. But guess what???? They are in the congress. When is the last time they were all able to talk to each other and make a useful decision. (Part tongue in cheek; part serious).
Personally; I would not take a chance of having each of them thinking that the other one is giving you the nomination, and them pulling their nomination. I don't trust that they'll communicate. And for the record; there isn't any "LAW" that says you have to tell them. It is nothing more than a courtesy. Well; if they did their job properly; or at least their staff who normally takes care of the academy nominations for them; then they'd contact their "Fellow Congressmen" and ask for a list of who they have nominated. This way, they could take it upon themselves to maximize their efforts if that's what they want.
In other words, it's not your job to do "THEIR JOB". Remember, they WORK FOR YOU. They are YOUR EMPLOYEE. If they want to know if you/GF's Son has a nomination, all they have to do is have one of their flunkies place a quick call or email. There's only 29 representatives in the state of New York. I'm positive that Schumer and Hillary's replacement has these 29 as an Email group. It would take them 39 seconds to ask who they've nominated. So I wouldn't tell them anything. They know how to find the answer if they want it. Your GF's son needs to take care of himself. Now; if he was being interviewed for a nomination; and he already had one prior; and they asked, then I would tell them. But if the nominations came after ALL the interviews were done; then I wouldn't say a thing. But that's just me. It's not a morals, values, or ethics decision. It's a "I don't believe 100% that the senators and representative will do their job properly and talk to each other.... and I am not willing to risk losing a nomination and possible appointment because of their mis-communications. And if they really want the information, they can it faster". Best of luck. mike.....