He's a powerful wizard and a talented duelist, but he's still not yet twenty. She knows not to underestimate him, but she is confident she can at the very least incapacitate him. She doesn't know the location of his own diary, but assumes it is in his room above Borgin and Burke’s or stashed away at the Lestrange manor (his connection to Abraxas not yet as strong as his influence over his son will be). She could overpower him with sectumsempra, he'd likely be surprised enough by the novel curse that she could sever the ring from his hand and apparate away with it. 