Maybe if he is a US resident and his M&M account is registered in the US they won't bother too much by the fact that his origin is DE and his destination is US. I've had similar issues a couple of times in ATH (while the origin of my trip was ATH ), booked however through lh.com, the US website) and I didn't have any problem to get 100EUR for same day expenses, without the need of returning receipts. Granted last time this happened was a couple of years ago, things may have changed meanwhile.

That said, If I were the OP I would definitely keep all invoices as totti suggests. If I had the time I'd go to FRA to check with some real person what is going on. As I said, 2 days still not in the system has only happened to me twice. Once, it was on AF (so it doesn't count, I've had the worst of the worst experiences with AF and LaPoste , things you can't imagine but is OT here) and the other on US Air. In the last case they never found my bag. Couple of months later they mailed me a check with 1000 USD