All trainers are not equal- some basically suck? If he saw a video and said use a belt Id right there say- She needs to work on form- not rely on a belt.
Research has shown that weight belts may not protect the spine to the degree once thought. They may stabilize the upper body by increasing pressure in the abdominal cavity.
If people wear belts much of the time the lower back muscles get bound which prevents the development of strength which is what weight lifters are after so belts become counter productive. When one starts to rely on a belt then your abdominals are not working
However- If she has a back injury or if she is training for an olympic meet or powerlifting meet or just lifts heavy then yes. However it sounds like she needs a good trainer that will help her improver her form NOT ADVISE TO USE BELT.
Try to find a trainer that has a more credible certification or has been training for long time- The guys at my gym, including myself we all have masters degrees and CSCS (Certified Strength and Conditioning Specialist ) which requires minimum of BA. Now believe me Im not one to always preach about that- because I trained for 20 years before I got certified and basically just so I could bust on all the ignorance surrounding the credibility of so many cerifications. Some people barely have ever trained in their lives went to a few weeks of classes and took a short test and abracadabra they become legal- thats a load of crap- Dont get me wrong in every arena and level you find ignorance but- sheesh- You dont fix bad form by using a belt. Thats like how so many newbies join to get big - hey Im 155 pounds want to begin lifting, what should I cycle with?? LOL She sounds like form is the problem.