No, it's not true!

Put simply, Type 1 diabetics can eat virtually the same types of foods as a non-diabetic can. All your boyfriend needs to bear in mind is that his insulin intake should balance against his food intake. Obviously, fruits contain fructose, a simple sugar, just as glucose is a simple sugar.

It makes little sense for your boyfriend to eat more food than he would do at any other time, and he should also bear in mind that a lot of fruits do have a high glycaemic index (GI). This means that his blood sugar will rise rapidly, but then come back down rapidly, dependent on the type of insulin he uses.

The best advice before you actually speak to a dietition is to take things in moderation.

You mention that your boyfriend "feels awful and dizzy some days". Is he testing his blood sugar levels at these times? It would pay him to do so as this might give some indication as to what, exactly, is happening. It's certainly something that he should mention to his doctor. If he's not testing his blood sugar levels he ought to be. Believe me, if he's not, he's more likely to develop complications (this, I can speak of through experience), and he won't like what he's headed for if he doesn't learn to control his sugar levels.

At the beginning of your question you asked about what foods to avoid. No doubt you're both intelligent enough to realise that foods that contain a lot of sugar are ones to avoid ... things like biscuits, sweets, cakes, etc. If you look on the food labels of most foods available in the UK you'll see the list of ingredients. Things near the top of that list means there's more of it in the product. i.e., if you see sugar near the top of that list, that means that there's a lot of it. He should also try to cut down on saturated fats.

I wish you both the very best of luck.