The person with whom you've been having lunch suddenly feels the call of nature and heads for the men's or ladies' room, or seems to have developed an acute case of arthritis that prevents him or her from picking up the check.
昼食をともにしていた人が急にトイレに行ってしまったり、勘定書きを手に取れないほどの急性関節炎を患ったみたいになったりとか。