Should someone stop Aricept (donepezil) and nemenda if they can still walk but not talk? No side effects, but could it contribute to mobility?

Meds for cognition. Aricept and Namenda (memantine) have been studied in patients with various types of dementia (alzheimer's disease, vascular dementia, lewy body dementia, etc.). Studies mostly assessed their effects on thinking, but they were also found to help activities of daily living and behavior. Therefore even if the patient can't talk, the medications could still be helping. Talk to the patient's md before stopping.
Individualize. There are three different scenario. One may be no difference while taking medications. One may be worsening of symptoms when stopping them. Or one may be improving symptoms when stopping them. The key factor is to control behavior and to maintain functional activity but not cognitive.
Med v. Disease. If you feel that the medicines are causing his deficit in talking or moving i guess you may stop them gradually. My feeling is that everything is due to his dementia even his lack of mobility and lack of conversation.
It depends. Not talking is not the same as not thinking or reacting to their environment. It is unlikely that stopping the two meds will adversely effect the persons ability to walk. Stopping them may, though, make them less "with it" in some sense. You could try for 2-3 weeks and compare and restart if you loose some efficacy you deem worth the price of the meds.