That roster does in fact include a Private Edward Irwin as a member of Company 'M'(page with printed number 293 in the pdf).
However, the gravestone inscription is slightly longer than in your question: ![Gravestone of B F Ervin](
SGT.
B. F. ERVIN
CO, D.
2ND IND. CAV.
Assuming that "CO, D." means Company 'D' in the roster, we can check that group's membership in more detail. There is no Ervin, Irwin etc, but there is a "First Sergeant Frank Ewing": ![Excerpt from roster of Company D](
It is not quite the right name, but he is the right rank in the right company. Therefore, given the scope for misprints and variant spellings, I think he is worthy of further investigation, if only to eliminate him.
The roster says that he died in 1862, while the grave has no death or burial date attached. If you can find when B. F. Ervin died, then you'll have a useful point of comparison with Frank Ewing.