Considering the fact that you've come a long way with your best friend and your mum cannot afford to let go of your best friend 'cos she needs someone around her as a woman who lost her husband, who would you prefer to lose; your mum or your best friend?

Some parents sef, why would your mum be going out with your best friend in the first place, not to talk of the age difference. Talk to your mum, she really has to stop the whole childish relationship and start being a mum instead. Nobody is saying she shouldnt date or be involved but she should find her agemate and if nothing else someone who is not her child's (best)friend, its degrading and irresponsible, where's her self-respect? I'm not trying to be rude or disrespectful to your mum, don't get me wrong but what she's doing is unacceptable and you should make her see/understand that,ok??  you have to stand up to herooo

whoa,

she can date any other guy for all i care but definetly not my best friend

Absolutely nothing!
If she dates my best friend, so what? As far as it is not my boyfriend.
I cant do anything afterall she's got her life to live.