First point, you have to charge GST on invoice issued to your client for reimbursements.
Second, your client is right. You have to charge GST on principal amounts only because you can take credit of GST charged on expenses.
But few expenses on which you cannot take input credit you to charge GST on entire amount.
For example, in your case:
For photography suppose 100 Rs. charged with 18 Rs. GST. So total invoice you received will be for 118 Rs.
You need to charge to your client 100+18. The 18 which you paid to photographer as GST, anyhow you can claim as input credit so it is not an expense for you.
However the expense you incurred on food, you need to charge full amount including GST and charge GST on that, because for food you cannot claim input tax credit.
Hope it solves your concerns.
chatCommentsThanks a lot. - Umesh Saxena