My 85 year old father received 2400 stimulus check. my mother died in 2018. does he need to return part of this? will he be penalized next year?

[DoninGA]

Yes, the stimulus payment portion for the deceased spouse must be returned.

See this IRS website for information (Question 10) - https://www.irs.gov/coronavirus/economic-impact-payment-information-center

A10. No. A Payment made to someone who died before receipt of the Payment should be returned to the IRS by following the instructions in the Q&A about repayments. Return the entire Payment unless the Payment was made to joint filers and one spouse had not died before receipt of the Payment, in which case, you only need to return the portion of the Payment made on account of the decedent. This amount will be $1,200 unless adjusted gross income exceeded $150,000. 

See this IRS website for information on how to return the payment (Question 52) - https://www.irs.gov/coronavirus/economic-impact-payment-information-center#more

[MinhT1]

Yes, your father needs to return your mother's share of the stimulus ($1,200).

Please read this IRS FAQ, especially questions 10 and 41 for more information.