Is a mourner allowed to sit in a regular chair while eating a meal – both seudas ha’vraah and all other meals during shiva?
The mourner sits on a low chair (less than 3 tefachim high) like Tisha B’av for both the seudas havra’ah and all of the other meals eaten during shiva, except for shabbos. If the avel is not well, and can’t sit on a low chair then he can sit on a regular chair
Y:D 387-1, Nishmas Yisroel chap. 4-6& pg, 343.