Need some advice...

Okay so this just happened last night. One of my best friends calls my girlfriend at like 10:00 p.m and Tells her that he loves her. He has felt this way for almost 2 years and couldn't bear it any longer without her knowing. She doesn't even know what to say on the phone so she kinda just tries to act like she has to go. I myself don't even know what the hell to do because he was 100% serious. need advice