You can surely take a look at what the others have mentioned already with regard to journaling food intake and logging exercise sessions. As trainers, we also have to look beyond the nutrition and exercise elements to see what is going on in their home and work environment as well. Support at home, stress, sleep patterns, and hormones can also affect weight loss.
Weight loss for many clients is a huge emotional journey… I would handle the weight gain by making sure the client knows I support them 100%.
You do want to provide honest feedback, but the client needs to know they have your trust and empathy.
It also helps to keep the big picture in mind, and praise your client for the little things. It’s those little changes that make a huge impact on health.