My mum is reaching 55 years old soon. Will she be forced to withdraw her CPF? If not, can she choose to leave it in the CPF account and let it compound over time till old age?

No no... no one will force her to withdraw from her CPF. If I were her, I would shift all I could (minus a nice holiday) into the RA to reap the high

returns of 4-6%

When she turns 55, her SA and OA will be set aside to meet the Full Retirement Sum in her newly opened RA Account. SA will be withdrawn from first before OA.

This forms her CPF Life from 65 for life.

Now if she has any balance above the FRS, she can withdraw this at anytime. It's almost like an ATM and you can do so via PayNow.