Helen's place is in a great location and I highly recommend staying here. Montgomery St. is located close to plenty of quieter restaurants, bars and shops and is also a short walk from nightlife if you're interested in that. You can get to any of the major tourist sites (Arthur's Seat, the Castle) within 15 minutes, and there are grocery stores and ATM's nearby as well.
Helen is an incredibly accommodating host who is friendly, flexible and great with staying in touch. I highly recommend staying here!