Her case sounds complex. She needs a good planner who understands the German and US retirement systems, her tax situation, maybe currency hedging depending on where she decides to retire.
A relative of mine inherited a property in France, sold it, and by the time she extracted the cash back to the US, most was gone to taxes and legal fees. It was a really terrible experience. I wonder if there would be as much friction and illiquidity for your relative. These are the things the planner has to know about.
I’m not on the efficient frontier. I think markets are efficient, but on a span of time than exceeds my ability to endure at age 61.5. I use long term trendfollowing to manage risk. Gary Antonacci’s work exemplifies what I do.