She prefers tea / to coffee / because it is / more healthier.

'Healthier' is already a comparative, so 'more healthier' is a double comparative and wrong. It should be simply 'healthier'. The use of 'prefers...to' in the first parts is correct, which may mislead one into suspecting it. A comparative form already carries the sense of 'more', so adding another 'more' or '-er' is always redundant in standard English.