my friend said marathon but I'm not sure so I want to double check.

Bermuda grass with a little nut sedge mix in.

If that is your lawn, it appears to be just fine , regardless of grass type.
Why mess with it?

Marathon III is the state-of-the-art dwarf fescue combining slow growth and a rich, dark green carpet-like appearance. However, because of its slow recovery from injury, I do not recommend it for everyone. It won't grow here in TEXAS because it can't take the heat.
You have Bermuda grass in the photo and not Marathon.