What is the best way for a new system to communicate with an Asp .NET system?

I have a client that is looking for a new site that communicates with the database of a system that it already uses in the company (implemented in Asp .NET). It is important the integrity of the data that will be displayed on the site since it is selling products and we can not sell a product that is no longer in stock. He talks a lot about security (because it can be attacked by competitors) but I do not know how to best meet the demand. I thought of a separate PHP system but access the same database, but I still do not know the best way to do that.