Sounds like preposterous advice from a dietitian, right? Not so fast.
In our "more-is-always-better" culture we tend to go to extremes. Since we have been told that vegetables are good for us it is easy to assume that we should always be striving to eat more of them.
If a person is underweight and needing to eat more to gain weight, vegetables aren't doing him much good. In fact, they might be getting in the way. It takes a lot more food than you might realize to gain weight for someone in this situation. A young male might need upwards of 5000 calories per day to return to a healthy weight range. Eating an entire pound of broccoli is only about 150 calories. It would take a lot of vegetables to get enough calories!
Eating a large number of calories to gain weight is often accompanied by gastrointestinal distress. The body isn't used to digesting that much food, so the sudden influx causes a traffic jam of sorts. After a few weeks the digestive process has sped up, but during that time things can get pretty uncomfortable with bloating, fullness, and constipation. Adding extra bulk and fiber with veggies doesn't help.
Thus, during the refeeding phase I typically recommend that clients limit their vegetable intake and focus on more calorically dense foods. And if choosing to consume vegetables I suggest adding calories to them with butter on corn, cheese sauce on broccoli, or ranch dressing with carrots. If having a salad put lots of caloric toppings on it. This is what it takes to gain weight, and sometimes it makes more sense to just take a hiatus from veggies for a while.
I always promise the client that nothing bad will happen if s/he isn't eating vegetables for a short period of time. In fact, if you never ate another vegetable again you'd be just fine. We make them out to be the holy grail of food, but it's still just food, just nutrients. There are plenty of people who live long healthy lives with little to no vegetable consumption. Can vegetables offer benefit to our health? Absolutely. But there are times that they can also be counter-productive.