Unwell 18 year old for TCs but not UC

Hi Team

Grateful for thoughts. Household has an 18 year old who has attended college very intermittently with long term health problems and PIP.  She is/was included in the tax credit award. Mum got a Migration Notice and claimed, and put the 18 year old as a non-dep due to the extended absence from college. I know UC does not have the same flexibility around extended absence due to illness. Does placing her as a dep or a non-dep make a difference in how UC would actually consider her position? And would it impact the TE calculation.

Thank you