Unless the reservation is updated to be properly in your name it shouldn't really post (it may, but not guaranteed - a basic name check would be a minimum step from Bonvoy). To have certainty on this you'd need the room to be updated in your name fully (which as long as your friend is fine with), which should be easy for the front-desk to do. In fact when booking multiple rooms I normally have the opposite problem - i.e. the front desk adds the guest names from passports/IDs, which causes problems with posting and I need to retro-claim.

Note, your friend may have hoped to get the extra points credit through this route as it is odd that they just add you as a second guest - whether they deserve it or not is another matter - to my mind, if you are paying for the room (another Marriott Bonvoy check/requirement is that you make the payment with your card), then you should be entitled to the points/nights credit.

If you aren't paying for the room, and you remain as only a named guest, then in theory it should not post - and there should be no route for a retro-claim. If you pay, and are the named first guest, it should post and if it doesn't then you could do a retro-claim with the receipt easily too.