I did a little date checking. (The magic googlefinger strikes again!)
Benjamin Franklin died 1790.
Thomas Jefferson died in 1826.
Beethoven was born 17 Dec 1770, died 26 Mar 1827.
OK, there's some overlap. For the Prof's story to work, Beethoven would have had to compose the concerto before Franklins death in 1790; Beethoven would have been no older than 20 then. I don't think he was all that famous yet, but the Prof's story is not yet impossible. I'm beginning to disbelieve the "visit" story, but I don't know it to be a tall tale.
The date I find for the composition of the Triple Concerto (Opus 56) is 1803/1804. The Prof's story about the visit and the Concerto is getting less and less plausible. Either it was a long-delayed inspiration, or Franklin haunted Beethoven.
I think the Prof was pulling your leg.