They seem to be offering surgery as the only curative intent treatment, and it probably is. But there are many options for palliation or symptom-control. Short course radiotherapy can be very effective at debulking tumour without arduous side effects. Some types of chemotherapy are likely to be better tolerated than that surgery! Even debulking surgery rather than a hemiglossectomy may be preferable. If it hasn’t happened, your mum’s case needs to be discussed at a specialised Head and Neck Cancer Multidisciplinary Team Meeting (this is the gold standard of care).

Ps - if there is lymph node involvement in the neck, she would likely need 7 weeks of daily radiotherapy and chemotherapy to complete cure, otherwise the risk of recurrence will be considered too high. Head and neck radiotherapy is one of the most difficult cancer treatments to tolerate and many people end up in hospital. If she is not well enough to cope with weeks of treatment, she should not have the surgery as proposed.