Your friend doesn't know much about the Navy.
The Air Force needs bases from which to launch strikes. It needs the men and equipment at those bases on a permanent basis to maintain the aircraft.
The Navy has aircraft carriers that are equipped with everything the Air Force has. And it can send an their "bases" wherever they're needed.
The Air Force has huge cargo planes for transporting men and equipment rapidly. The Navy leaves that to the Air Force. They have a few workhorses, but they're small.
You're friend says the Marines can do all the necessary fighting. I think the Army would take exception to that. The Marines are designed for a high-intensity, low profile assault. The Army is in there for the long haul. Their missions are different.
You allude to the clandestine missions carried out by SEALs. While the other services have their special forces, if the job is anywhere near the water, the SEALs are best equipped to handle it.
And, I guess your friend has not heard of our submarines. The Navy has subs each with more firepower than some nations. And, like the surface ships, they can go anywhere. Being under water makes them hard to find. Being mobile makes them moving targets.
And, when we have to make our presence know, when we need to make a show of force, when we want to let somebody know that, "Guess what... we're watching you," we don't have to negotiate with a government to allow us access to their land; we don't have to build a base for the Air Force, or to house a couple thousand troops. We send in the Navy.
An aircraft carrier task force is an awesome array of weaponry. It has missiles, it has a small contingent of Marines. It has a air wing consisting of the specific aircraft needed at that place and time. It has all the facilities of a land base... and then some. And it's there... now. There's no "ramp up" time. When it arrives, it's ready to operate. If there's a need to field a large number of troops for the long haul, the Air Force is called in and they build the strips on which to land and from which to take off, establish the support facilities, and begin moving in personnel and equipment. And the Navy will cover them while they put their planes and personnel in place. And the Navy can configure its task forces to meet the needs of the situation. And it can change that configuration rapidly.
Is the Navy done, then? Nope. Depending where they are, and where they are needed, SEAL's take off from carriers, are released from submarines, or are dropped from Navy planes. They take off from land bases too. The Navy also has a few of them left.
Over 70% of the Earth is covered in water. That leaves less than 30% for land. The Army and Air Force are designed to work from that 30%. The Navy can operate from everywhere else.