Instead of asking us, perhaps you should be asking your client what they would prefer. They're the ones paying you for your services.

Be direct with them and tell them the light died faster than you expected and it affected the quality of the photos taken near the end of the session. Give them several options as to how to proceed from the following possibilities. Only give options with which you are comfortable.

• Do a reshoot at no additional charge to them
• Give them a discount on the session and submit what you have to them including your best edits of the lower quality images
• Give them a more substantial discount and only supply the images that meet quality standards with which you are comfortable
• Give them a full refund with the understanding no images will be delivered
• Give them a full refund and give them a few of the images that meet your standards and offer to give them a discount on a future session

There's no way that you can predict what their response will be, so only offer options with which you are comfortable.

Another session may or may not be convenient to them (perhaps some family members were only in town for the session?). Show them the best processing you can do with the shots in question and see if they are acceptable to them. They may be totally happy with them, they may prefer them to nothing if the session can not be redone, or they may decide they want to take their business elsewhere.

Whatever their response, be professional and continue treating them as a valued customer.

Then learn from your mistake going forward. One thing a lot of photographers do if at all possible is to take the large group photos first when a session has a mixture of large groups, smaller groups, and individuals. Another thing to consider, if you don't already, is to have off-camera lights available to mix with the natural lighting for late afternoon outdoor sessions. You can get some real magic with well used flashes during the golden and blue hours!