Build a bed

My husband is going to build a bed for our son. We bought him a bed already and put in his bed room but he does not like sleeping in it. So my husband plans to build a bed for our son that he will like but he told me that we will need to buy a new mattress. I do not want to buy a new mattress because I know that a mattress is not cheap. My friend told me that she had a full size mattress from her son’s bed that is in her garage because she bought a new one and she is going to give to us. She is so nice to us, I am hoping that she does not change her mind and she will wait until my husband is finished with the bed.