Does Mastectomy Worsen Dementia?

my 91 yr old mom with later stg dementia has idc , the full path report is not back yet. surgeon wants to do a mastectomy but i know her dementia will worsen.my sis and i do not know wat to do. her tumor is very large with swelling on bottom of breast. she eats good and sleeps most of the day. she is very content at this point in her life. we dont know wat to do re the mastectomy