My mother’s teeth have yellowed since having the 6 month smiles treatment, should she speak to her dentist?

Q.
Hi. could you please help me. My mum has just had 6 month smiles. she had a ceramic brace on the front of her teeth. to be fair the treatment worked both fine and quickly, I personally am impressd with the results she got and am considering it myself now. the issue she’s having is that her teeth were never really a bright white but they’v got slightly more yellow. despite me being adamant they’re still OK so long as her gums are she want them whitened. this is what I need help with, I have told her to return to the dentists where the treatment was done and have it done there as their work has already been proven with the brace. she however is insisting on trying to save money and has seen an offer at a place that offers a few ‘beauty treatments’ (I wouldn’t say a beauty salon but If i did you’d know the type of place i mean). could you please advise on the fact that if its that cheap there’s a reason! Im not a fan of these places myself. thanks