💡 Detailed Explanation
The correct choice is Option 1: 'Really? Then, shall I lend you some money?' When a friend is in trouble because they forgot their wallet ('財布、忘れちゃった'), offering to help by lending money ('私が貸そうか？' - Shall I lend it [to you]?) is the most helpful and natural response. Option 2 ('じゃあ、忘れないでね' - Then, don't forget it) is an advice after the fact and doesn't help the immediate problem. Option 3 ('じゃあ、ありがとう' - Then, thank you) makes no sense because the speaker has not done anything to be thanked for.