As a practical matter, you should have an attorney assist you in determining who is responsible for ADA compliance under your lease before you acquire the restaurant.
More important, you do not have the right to have your customers use another restaurant's bathroom. Further, having your customer's go to another building to use the restroom is probably not compliant under the local building code. Finally, unless your restaurant considered "take out" only, you will probably required to have a bathroom for your customers. You should contact the local department of building and safety to determine the applicable requirements.