My mum was diagnosed with pleomorphic parotid adenoma and she has a heart condition. is an operation really necessary? my mum is 79

An operation: is generally necessary. Ask your doctor whether or not her medical risks outweigh benefits of surgery, because a lot depends on the severity of her heart condition.

Possibly: Pleomorphic adenomas of the parotid do convert to carcinoma, and the first sign could be facial nerve paralysis. If her medical condition is good, and you have a good surgeon/oncologist, the operation with facial nerve dissection is a 45 minute procedure with an overnight stay.