If they cannot enter Germany, skip the First Class Terminal. But they can still use the Lufthansa First Class Lounge in FRA Pier B airside before a US flight. With wheelchair assist and a 3 hour layover, they’ll have time to clear security, rest, eat, and get escorted to the gate.
Is First that much better than Business on LH? It is nicer for privacy, food, and service. But LH Business is already a lie-flat and comfortable. For 50k more miles and about $100, I’d only do it if the second leg is overnight or you really want to spoil them. Also consider the risk of splitting seats on DEL to FRA if you rebook.
If it were my parents, I’d keep the current business itinerary unless seats together are guaranteed on the long first segment and the First segment lines up with a good rest window.
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