In Oni's defence he did say mathematical equations are all reversable and whilst those examples do use mathematical equations you are really stating the irreversable processes not the maths involved. At least provide a proper retort if you are trying to make him look like an idiot.
Not trying to make him look like an idiot, just showing there are examples that he could not think of. Was having a laugh with the steak one though =)
Heat loss, as used in house insulation (somewhat related to how Pink and Green batts get their R rating)
Q = U*A*ΔT
Combustion of Ethanol, I run pure eth or E85 in my cars
C2H6O +3*O2 --> 3*H2O + 2*CO2
Glycolysis, phosphorylation steps are irreversible
Glucose + 2*ADP + 2*NAD+ --> 2*Pyruvate + 2*ATP + 2*NADH
C6H12O6 + 2*C10H15N5O10P2 + 2*C21H27N7O14P2 -->
2*C3H3O3 + 2*C10H16N5O13P3 + 2*C21H29N7O14P2
Honestly don't know one for friction, have always done work within the assumption of vaccuum/no friction. But I'm sure I could find one somewhere.