What if sponsor for my mother has an expired passport?

Hi everyone!)

I have been looking for help in many forums, but... no answers...(((

Can some kind people give me a hand?! please)))

the question is actually about my mother sponsor's. As this will be second time when she coming in UK by Family Visitor visa I need to enclose supporting document from sponsor.

Last her visit as a sponsor we put my husband' mother (becouse I am full-time mother no working, my husband has a low income, only his mom are long day worker in a good income to be a sponsor).

was nop at first time, but now there is... My husband' mother has an expired passport, sd to be a sponsor we must to send a copy of her passport. but it takes a crazy long time with all Renewal proccess( I therefore believe that expired documents, such a passport are invalid documents, am I right? What is the best way to act, let me know, please... confusing...(((

thanks for anyone's attention!

You need a valid passport with at least 6 months left in it when you apply for the visa

thank you)))