All you need to do is keep to the basic guidelines you learned on your PT course unless you have completed another nutrition based course like precision nutrition certificate. This certification will give you a wider scope on nutrition to help your clients with there dietary needs, so check it out if you wish to get more in depth with nutrition (link below). Secondly, in regards to going shopping with your client to provide basic nutrition guidance should be within your scope as a personal trainer but be careful and stay within your scope of knowledge by sticking to the guidelines of your PT course you studied.
Hope I was helpful in answering your questions 🙂
Wish you all the best in health and wealth