Of course a girl and a guy can be friends! And a straight girl and a straight guy can be friends too! For this to work well, the situation has to be clear to both of them.
The basis for friendship is to have things in common, trust each other, support each other, and have fun together. All these things are possible between a girl and a guy. However, it’s important not to be naive: the situation can change. For example, one of the two might fall in love with the other, start to feel desire for the other, or get a boyfriend or girlfriend. All these events, and many more, can change the context of a guy-girl friendship.
You have to be honest from the start and reassess the situation from time to time. In that way, you’ll always be certain of being on the same wavelength so nobody gets hurt in the friendship.