It is common for there not to be any seats available for preselection for single travellers on a Qsuite aircraft, as only 16 out of 42 are made available to them for preselection. And once they're gone you have to wait until either online check-in or someone else cancels. Or call up.

If MMB doesn't give any way to see or change a seat, the best thing is to call or use the live chat. They will likely be able to confirm a seat there and then, and/or process a refund if the offered seat position is materially different to what was paid for. That might be a win for your friend if they now don't mind where they sit in a Qsuite layout