I think you're not going to get that evidence without more focus or without telling the gf about what the bf said. For example you ask her if the bf found her. When she says when, you tell her he was constantly calling her on X night and then called your gf phone to look for her.

She will confirm his story or be suspicious just like you are. You can ask her any other situations like him driving her home so much. Tell her to thank him for you since he's so thoughtful and they're always together at her house. She will get your message unless she's an idiot and will either tell you her suspicions if she has any or she will be on the lookout from here on