Yes my friend! First, you need to create a CSS, with a Partition that is assigned to your Manager line. Assign that CSS to the CSS-SUBSCRIBE field on the phone and/or UDP (in case of Extension Mobility).
Then you create a Phone Button Template where you have the BLFs (the option is BLF Speed Dial). Then assign that new Phone Button Template to the phones you want to have BLF on.
When you change the phone button template, assign it to the Assistant's phone and configure her BLF Speed dial by clicking on it.
Don't forget to put that CSS you created in the beginning in all CSS-SUBSCRIBE fields. In case of Extension Mobility, you assign the CSS-SUBSCRIBE in the User Device Profile, found in Device > Device Settings > Device Profile.