I have never seen anyone due this on any flight I have ever been on and pray I never do. I do find it gross, but don't think the FA could do anything about it. The only time I have heard of them taking nail clippers is if they have a file attached to them, and I haven't heard of that in a while.
I found it disgusting last month on DL when I was flying in in seat 1c and the person in 1d (or I was in 1B and it was the person in 1A), took off their shoes and put their bare feet on the wall. I guess that wasn't so bad compared to what you encountered.
DL and occassionally WN are the only airlines I fly anymore, and find the service in WN to be better than coach on Delta.