@cor3zone
Well the suggestion to go to the beach was originally made by his friend. I think what makes the most sense is that:
-He's invited to the beach
-He tells his friend he wants to invite his gf
-He asks if she has plans
-She says yes
-He doesn't mention the beach at all because she confirmed she's busy
-One of her friends suggests going to the beach
-They're surprised by each other being there