That must be really hard for you. I've worked with a mum with bulimia and she was always worried about what her children thought.
Are you close to your mum? Why don't you talk to her and just say that you realise what she is doing to herself, and that she should go to the doctors who can refer her onto an eating disorders service.
Does she binge eat too or is she more restrictive with her eating (or she may be totally normal in her eating). I only ask as it may not be bulimia, some anorexics purge too, but unless she is very underweight then she wouldn't be classed as anorexic.
In the mean time, try spending time with her after she has eaten to take her mind off the food. You're 100% right, in that vomiting does not work as a form of weight control, nor do methods such as laxatives. It could be seriously damaging g salivary glands, her gullet and her stomach, not to mention her teeth. But bear in mind people with eating disorders who do not want to be helped, can't change. She will have to want to change, in order for treatment to be offered.