Assuming you can't afford Depends, Then bleach is your friend. If mom doesn't use white cotton briefs, buy her some, they're very inexpensive through Hanes on line or probably WalMart. Wash then in a small load hot water. Add a cup of bleach and detergent. Fill the washer, let it agitate a few minutes. Turn it off and let the, ask an hour. To collect nigh to wash, put them in a plastic bag and twist tie it shut. Use a fabric softener sheet or two in the dryer.
Go up to the upper right corner of this page where you will see "search site" and type in "urine clothes" and you will get articles and comments on what to do. Good luck.