My mother is suffering from mantle cell lymphoma. Tell me how bad this is?

Serious. Mantle cell lymphoma is a serious type of non-hodgkin lymphoma. In a younger patient under the age of 70 and otherwise in good health it can be put into remission for several years with chemotherapy and stem cell transplantation. In older patients or patients who are otherwise sick there are some treatments i can put the disease into remission for a few years.
Depends. Mantle cell lymphoma is responsive to but probably not cured by conventional chemotherapy. What the best treatment is, when it should include stem cell transplant, is uncertain. If available , participate in a clinical trial. Some patients who are favorable can live beyond 5 years, but in others it is an aggressive disease that doesn't do well.