a theistic friend told me he has two arguments against atheism
1- “God is like love: you cannot prove it exists, but you know it is there”
2- “There are many things we cannot see, but we know they exist: air, bacteria, atoms, etc. It is the same with God. ”
How to refute these arguments? 
Those are dumb arguments in favour of the existence of a god.

1) As Davin said, love is an emotion, and exists as a subjective experience inside the head of people and other beings experiencing it, it does not exist outside of the brain.
2) All those are very easily measurable and exist in the natural world.

Quite frankly, that was the stupidest argument I've ever heard.
Take air for example, air has weight and mass. You can blow it into a balloon, fill your lungs with it...there are countless ways to demonstrate that air exists. You can actually see bacteria under a microscope. And heck, Germ Theory is just a theory, right? You can also measure atoms and "see" them using specialized instruments.
That argument is just so inane I don't even know what else to say.